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United States Patent |
6,177,166
|
Ohno
,   et al.
|
January 23, 2001
|
Optical information recording medium
Abstract
An optical information recording medium comprising a substrate, a recording
layer, a protective layer containing a sulfur atom, an intermediate layer
in contact with the protective layer and a reflective layer containing
silver as the main component in contact with the intermediate layer,
wherein the intermediate layer comprises an element which does not form a
compound with silver, the element contained in the intermediate layer
having a solid solubility of at most 5 atomic % to silver and silver
having a solid solubility of at most 5 atomic % to the element contained
in the intermediate layer, on the side in contact with the reflective
layer, and the intermediate layer comprises an element less reactive to
sulfur or its sulfide comprises chemically stable elements, on the side in
contact with the protective layer.
Inventors:
|
Ohno; Takashi (Yokohama, JP);
Komatsu; Masao (Jurong, SG);
Nobukuni; Natsuko (Yokohama, JP)
|
Assignee:
|
Mitsubishi Chemical Corporation (Tokyo, JP)
|
Appl. No.:
|
192321 |
Filed:
|
November 16, 1998 |
Foreign Application Priority Data
| Nov 17, 1997[JP] | 9-314914 |
| Nov 20, 1997[JP] | 9-319550 |
Current U.S. Class: |
428/64.1; 369/283; 369/288; 428/64.4; 428/64.5; 428/64.6; 428/457; 428/913; 430/270.13; 430/495.1; 430/945 |
Intern'l Class: |
B32B 003/02 |
Field of Search: |
428/64.1,64.2,64.4,64.5,64.6,457,913
430/270.13,495.1,945
369/283,288
|
References Cited
Foreign Patent Documents |
0 683 485 | Nov., 1995 | EP.
| |
0 779 614 | Jun., 1997 | EP.
| |
0 844 607 | May., 1998 | EP.
| |
0 867 868 | Sep., 1998 | EP.
| |
5-81719 | Apr., 1993 | JP.
| |
7-57301 | Mar., 1995 | JP.
| |
10-228676 | Aug., 1998 | JP.
| |
Primary Examiner: Evans; Elizabeth
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed is:
1. An optical information recording medium comprising a substrate, a
recording layer, a protective layer containing a sulfur atom, an
intermediate layer in contact with the protective layer and a reflective
layer containing silver as the main component in contact with the
intermediate layer, wherein the intermediate layer comprises an element
which does not form a compound with silver, the element contained in the
intermediate layer having a solid solubility of at most 5 atomic % to
silver and silver having a solid solubility of at most 5 atomic % to the
element contained in the intermediate layer, on the side in contact with
the reflective layer, and the intermediate layer comprises an element less
reactive to sulfur or its sulfide comprises chemically stable elements, on
the side in contact with the protective layer, wherein the reflective
layer has a silver content of at least 95 atomic %.
2. The optical information recording medium according to claim 1, wherein
the reflective layer comprises pure silver or a silver alloy containing at
least one component selected from the group consisting of Ti, V, Ta, Nb,
W, Co, Cr, Si, Ge, Sn, Sc, Hf, Pd, Rh, Au, Pt, Mg, Zr, Mo and Mn in an
amount of from 0.2 atomic % to 2 atomic %.
3. The optical information recording medium according to claim 1, wherein
the protective layer contains a sulfide compound.
4. The optical information recording medium according to claim 1, wherein
the recording layer comprises an alloy thin film of Ma.sub.w (Sb.sub.z
Te.sub.1-x).sub.1-w wherein 0.ltoreq.w.ltoreq.0.3, 0.5.ltoreq.z.ltoreq.0.9
and Ma is at least one component selected from the group consisting of In,
Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Pb, Cr, Co, O, N, S, Se, Ta, Nb,
V, Bi, Zr, Ti, Mn, Mo, Rh and a rare earth element.
5. The optical information recording medium according to claim 4, wherein
0.ltoreq.w.ltoreq.0.2.
6. The optical information recording medium according to claim 4, wherein
0.6.ltoreq.z.ltoreq.0.8.
7. The optical information recording medium according to claim 1, wherein
the intermediate layer comprises an element having an eutectic point of at
least 500.degree. C. in a binary alloy phase diagram with silver on the
side in contact with the reflective layer.
8. The optical information recording medium according to claim 7, wherein
the intermediate layer comprises at least one component selected from the
group consisting of tantalum, nickel, cobalt, chromium, silicon, tungsten
and vanadium on the side in contact with the reflective layer.
9. The optical information recording medium according to claim 8, wherein
the intermediate layer comprises at least one component selected from the
group consisting of tantalum and nickel on the side in contact with the
reflective layer.
10. The optical information recording medium according to claim 1, wherein
the intermediate layer comprises an element which does not form a compound
with sulfur or a compound with sulfur comprises elements which do not
cause decomposition, sublimation, melting and phase transformation at a
temperature of not higher than 500.degree. C., in a binary alloy phase
diagram with sulfur on the side in contact with the protective layer.
11. The optical information recording medium according to claim 10, wherein
the intermediate layer comprises at least one component selected from the
group consisting of aluminum, silicon, germanium, tantalum, nickel,
cobalt, chromium, tungsten and vanadium on the side in contact with the
protective layer.
12. The optical information recording medium according to claim 11, wherein
the intermediate layer comprises at least one component selected from the
group consisting of aluminum, silicon, germanium, tantalum and nickel on
the side in contact with the protective layer.
13. The optical information recording medium according to claim 12, wherein
the intermediate layer comprises at least one component selected from the
group consisting of aluminum, tantalum and nickel on the side in contact
with the protective layer.
14. The optical information recording medium according to claim 1, wherein
the intermediate layer comprises at least two layers including a layer
containing aluminum as the main component provided in contact with the
protective layer and a layer of preventing diffusion of aluminum and
silver provided in contact with the reflective layer.
15. The optical information recording medium according to claim 14, wherein
the layer of preventing diffusion of aluminum and silver comprises a
compound of aluminum or silver with oxygen and/or nitrogen.
16. The optical information recording medium according to claim 15, wherein
the layer of preventing diffusion of aluminum and silver comprises a
compound of aluminum or silver with oxygen.
17. The optical information recording medium according to claim 14, wherein
the layer of preventing diffusion of aluminum and silver comprises at
least one component selected from the group consisting of tantalum and
nickel.
18. The optical information recording medium according to claim 14, wherein
the layer containing aluminum as the main component has an aluminum
content of at least 95 atomic %.
19. The optical information recording medium according to claim 18, wherein
the layer containing aluminum as the main component comprises pure
aluminum or an aluminum alloy containing at least one component selected
from the group consisting of Ta, Ti, Co, Cr, Si, Sc, Hf, Pd, Pt, Mg, Zr,
Mo and Mn, in an amount of from 0.2 atomic % to 2 atomic %.
20. The optical information recording medium according to claim 1, wherein
the intermediate layer comprises at least one component selected from the
group consisting of tantalum, nickel, cobalt, chromium, tungsten and
vanadium.
21. An optical information recording medium for recording, retrieving and
erasing mark length-modulated amorphous marks by irradiating with a
focused light beam, which comprises a substrate, a lower protective layer,
a phase-change recording layer comprising an alloy thin film of Ma.sub.w
(Sb.sub.z Te.sub.1-z).sub.1-w (wherein 0.ltoreq.w.ltoreq.0.2,
0.6.ltoreq.z.ltoreq.0.8 and Ma is at least one component selected from the
group consisting of In, Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Pb, Cr,
Co, O, N, S, Se, Ta, Nb, V, Bi, Zr, Ti, Mn, Mo, Rh and a rare earth
element) having a film thickness of from 10 nm to 30 nm, an upper
protective layer having a film thickness of from 30 nm to 60 nm, a first
reflective layer containing aluminum as the main component and having a
film thickness of from 5 nm to 50 nm, a diffusion-preventing layer in
contact with the first reflective layer, and a second reflective layer
containing silver as the main component and having a volume resistivity of
from 20 n.OMEGA..multidot.m to 80 n.OMEGA..multidot.m and a film thickness
of from 40 nm to 200 nm in contact with the diffusion-preventing layer.
22. The optical information recording medium according to claim 21, wherein
the first reflective layer has a volume resistivity of from 20
n.OMEGA..multidot.m to 150 n.OMEGA..multidot.m.
23. An optical information recording medium comprising a substrate, a
recording layer, a protective layer containing a sulfur atom, an
intermediate layer in contact with the protective layer and a reflective
layer containing silver as the main component in contact with the
intermediate layer, wherein the intermediate layer comprises an element
which forms a continuous series of solid solutions with silver, wherein
the reflective layer has a silver content of at least 95 atomic %.
24. An optical information recording medium comprising a substrate, a
recording layer, a protective layer containing a sulfur atom, an
intermediate layer in contact with the protective layer and a reflective
layer containing silver as the main component in contact with the
intermediate layer, wherein the intermediate layer comprises amorphous
carbon or an oxide, a nitride or a carbide of a semiconductor or metal,
wherein the reflective layer has a silver content of at least 95 atomic %.
25. An optical information recording medium comprising a substrate, a
recording layer, a dielectric protective layer, an intermediate layer, and
a reflective layer containing silver as the main component in contact with
the intermediate layer, wherein the intermediate layer comprises at least
one component selected from the group consisting of tantalum oxide,
tantalum and nickel, wherein the reflective layer has a silver content of
at least 95 atomic %.
26. The optical information recording medium according to claim 25, wherein
the dielectric protective layer is a layer comprising ZnS--SiO.sub.2 as
the main component, and the intermediate layer comprises tantalum oxide,
and the reflective layer comprises silver as the main component.
27. The optical information recording medium according to claim 25, wherein
the intermediate layer comprises at least two layers including a layer
containing aluminum as the main component provided in contact with the
protective layer and a layer comprising tantalum or nickel provided in
contact with the reflective layer.
28. The optical information recording medium according to claim 25, wherein
the intermediate layer comprises a layer comprising an aluminum alloy
containing tantalum in an amount of from 0.1 atomic % to 2 atomic % and
having a film thickness of from 5 nm to 50 nm and a layer comprising
tantalum having a film thickness of from 5 nm to 50 nm and the reflective
layer having a film thickness of from 30 nm to 200 nm.
29. The optical information recording medium according to claim 25, wherein
the reflective layer comprises pure silver or a silver alloy containing at
least one component selected from the group consisting of Ti, V, Ta, Nb,
W, Co, Cr, Si, Ge, Sn, Sc, Hf, Pd, Rh, Au, Pt, Mg, Zr, Mo and Mn in an
amount of from 0.2 atomic % to 2 atomic %.
30. The optical information recording medium according to claim 25, wherein
the recording layer comprises an alloy thin film of Ma.sub.w (Sb.sub.z
Te.sub.1-z).sub.1-w wherein 0.ltoreq.w.ltoreq.0.3, 0.5.ltoreq.z.ltoreq.0.9
and Ma is at least one component selected from the group consisting of In,
Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Pb, Cr, Co, O, N, S, Se, Ta, Nb,
V, Bi, Zr, Ti, Mn, Mo, Rh and a rare earth element.
Description
The present invention relates to an optical information recording medium
such as a phase-change type recording medium and a magneto-optical medium.
Along with an increasing amount of information in recent years, a recording
medium capable of recording and retrieving a large amount of data at a
high density and at a high speed has been demanded, and an optical disk is
expected to be just suitable for such an application.
Optical disks are classified into a write-once type disk which is capable
of recording only once and a rewritable type disk which is capable of
recording and erasing for any number of times. As a rewritable type
optical disk, a magneto-optical medium utilizing a magneto-optical effect
or a phase-change medium utilizing the change in reversible change in the
crystal and/or amorphous state, may be mentioned.
The phase-change medium has a merit that it is capable of recording/erasing
simply by modulating the power of a laser beam without requiring an
external magnetic field, and the size of a recording and retrieving device
can be made small and simple.
Further, it has a merit that a high density recording can be attained by a
shorter wavelength light source without any particular alteration of the
material of e.g. the recording layer from the medium capable of
recording/erasing with a wavelength of about 800 nm, which is currently
mainly used.
As the material for the recording layer of such a phase-change medium, a
thin film of a chalcogenide alloy is often used. For example, an ally of
GeSbTe type, InSbTe type, GeSnTe type or AgInSbTe type may be mentioned.
Its layer structure is generally a quadri-layer structure comprising a
protective layer, a recording layer, a protective layer and a reflective
layer.
In a rewritable phase-change type recording medium which is practically
employed at present, a crystal state is an unrecorded or erased state, and
an amorphous state is a recorded state.
The recording, i.e. formation of the amorphous marks, is carried out by
heating the recording layer to a temperature higher than the melting
point, followed by quenching. The erasing, i.e. crystallization, is
carried out by heating the recording layer to a temperature higher than
the crystallization temperature of the recording layer but lower than the
melting point.
To prevent evaporation or deformation of the recording layer by such
heating or quenching treatment, it is common to sandwich the recording
layer with heat resistant and chemically stable dielectric protective
layers. In the recording step, these protective layers facilitate heat
dissipation from the recording layer to realize supercooled state, and
thus contribute to formation of the amorphous marks, and in the erasing
step, the protective layers work as heat-accumulating layers to maintain
the recording layer at a high temperature suitable for solid phase
crystallization.
Further, it is common that a metal reflective layer is formed on the above
described sandwich structure to obtain a quadri-layer structure, whereby
the heat dissipation is further facilitated so that the amorphous marks
will be formed under a stabilized condition.
A phase-change type recording medium one-beam overwritable simply by
modulating an intensity of one focused laser beam is erasing and
re-recording steps, is noteworthy as a medium for an inexpensive recording
system of high density and large capacity since a layer structure and a
drive circuit structure can be simplified.
Recently, a CD (Compact Disk) or DVD (Digital Versatile Disk, or Digital
Video Disk) has been developed by using such a phase-change type recording
medium.
A rewritable CD (CD-Rewritable, CD-RW) does not satisfy the standard of
present CD requiring a reflectance of at least 70%, but secures a
compatibility of groove signals and recording signals to CD in a
reflectance range of from 15 to 25%. Also, if an amplification system is
applied to a regeneration system in order to cover the low reflectance, a
compatibility can be secured within the scope of present CD driving
techniques.
CD-RW is provided with a wobbling groove, in which recording is carried
out. The wobbling frequency is one having a carrier frequency of 22.05 kHz
frequency-modulated (FM) by address information. Its wobble amplitude is
very small (about 30 nm) in comparison with a groove pitch (1.6 nm).
This is called as ATIP (Absolute Time In Pre-groove) signal wherein
wobbling is frequency-modulated and address information of a certain track
at a specific position is introduced.
ATIP signal is already used in a write once type disk (CD-recordable, CD-R)
with an organic dye. By using ATIP signal, it becomes possible to control
the rotational speed of an unrecorded disk, and recording can be carried
out at a linear velocity of 1, 2 or as high as 4 or 6 times of the CD
linear velocity (from 1.2 to 1.4 m/s).
Actually, a commercially available CD-R is generally a medium
satisfactorily recordable at a linear velocity of either 2 or 4 times of
the CD linear velocity.
Thus, it is demanded also with regard to phase-change recording CD-RW that
is satisfactorily overwritable at a linear velocity in the range of at
least 2 times (2.4-2.8 m/s) to 4 times (4.8 m/s-5.6 m/s), further in the
range of 6 times (7.2-8.4 m/s) to 8 times (9.6-11.2 m/s) of the CD linear
velocity.
On the other hand, a rewritable recording medium having a higher recording
density, i.e. a rewritable DVD, has been developed by using such a
phase-change recording technology. And also in this case, it is demanded
that the rewritable DVD is satisfactorily overwritable at a linear
velocity in the range of at least 2 times (7 m/s) and even to 4 times (14
m/s) of the read-only DVD linear velocity (3.5 m/s).
In such a case, in order to use an inexpensive semiconductor laser, it is
desirable that the recording power is at most about 15 mW, and even if the
linear velocity during recording is different, a desired or the same mark
length must be recorded with high quality, simply by changing the
reference clock frequency in inverse proportion to the linear velocity.
However, with a phase-change medium, if the ratio of the maximum linear
velocity to the minimum linear velocity for overwriting exceeds about 2,
it becomes impossible to carry out proper recording at either linear
velocity, in many cases.
Usually, a recordable disk requires a different irradiation power to heat
the recording layer to the same temperature for the different linear
velocity. Even if the maximum temperature of the recording layer is
brought to the same level by adjusting the irradiation power, if the
linear velocity is different, the same heat history including temperature
rising rate, cooling rate and temperature distribution may not necessarily
be accomplished.
Formation of amorphous marks during recording is carried out by quenching
the recording layer which has once been melted by heating, at least a
specific critical cooling rate, and crystallization during erasing is
carried out by relatively slowly cooling the heated recording layer. This
cooling rate depends on the linear velocity when the same layer structure
is employed. Namely, at a high linear velocity, the cooling rate is high,
and at a low linear velocity, the cooling rate is low.
Thus, as a linear velocity during overwriting becomes higher, a cooling
rate in the vicinity of a melting point becomes higher and amorphous marks
are easily formed. On the contrary, as the linear velocity becomes lower,
the cooling rate becomes lower and there is a fear that recrystallization
during recording tends to occur.
This is proved by the following simulation results made by the present
inventors.
Heat distribution simulation was carried out by solving a thermal diffusion
equation when applying with recording power and erase power with regard to
a disk having a protective layer (100 nm) comprising ZnS and SiO.sub.2, a
recording layer (25 nm) comprising Ge.sub.2 Sb.sub.2 Te.sub.5, a
protective layer (20 nm) comprising ZnS and SiO.sub.2 and a reflective
layer (100 nm) comprising Al alloy respectively formed on a polycarbonate
substrate.
A cooling rate in the vicinity of a melting point (600.degree. C.) was
estimated at a position of 0.1 .mu.m from the pulse irradiation-initiating
point during the temperature-descending process after reaching the maximum
temperature (1350.degree. C.) by heating a recording layer, and the
calculated results were 0.9 K/nsec at a linear velocity of 1.4 m/s, 2.2
K/nsec at a linear velocity of 4 m/s and at least several K/nsec at a
linear velocity of at least 10 m/s.
On the other hand, to erase the amorphous marks during erasing, it is
necessary to maintain the recording layer at a temperature higher than the
crystallization temperature and lower than the melting point or its
vicinity for a certain period. Accordingly, if a laser beam irradiation
for overwriting is carried out under a relatively high linear velocity,
the heat distribution at the irradiated portion of the recording layer
becomes relatively rapid timewise and spatially, whereby there will be a
problem that during erasing, crystallization tends to be insufficient and
a non-erased portion may remain.
To cope with such a recording condition, an alloy with a composition having
a relatively high recrystallization ability may be used for the recording
layer, or a layered structure whereby heat is hardly dissipated, may be
employed for the recording layer, so that crystallization i.e. erasing of
amorphous marks can be completed in a relatively short period of time. On
the contrary, under a relatively low linear velocity recording condition,
the cooling rate tends to be low as described above, whereby
recrystallization during amorphous mark formation is feared. As a method
for preventing recrystallization during amorphous mark formation, an alloy
with a composition having a relatively slow recrystallization ability may
be employed, or a layer structure whereby heat is readily dissipated, may
be employed for the recording layer. That is, two kinds of media must be
prepared depending on a linear velocity.
However, with e.g. CD-RW or a rewritable DVD, it is not preferred that
separate disks have to be prepared for recording at 2- and 4-times
velocities of CD or DVD.
In order to solve this problem, with regard to a phase-change type
recording medium having a GeTe--Sb.sub.2 Te.sub.2 pseudo-binary alloy type
recording layer, there have been some reports including one by the present
inventors with respect to a method for obtaining good overwriting
characteristics within a linear velocity range of from about 1 m/s to
about 10 m/s by changing the pulse strategy during overwriting (a system
for controlling by dividing the irradiation beam into pulses to obtain a
good pit shape) depending upon the linear velocity.
However, generally, to implement a variable pulse strategy makes the pulse
generation and laser driving circuit, etc. complicated, thus leading to an
increase of the cost for producing the drive. Accordingly, it is desirable
that a wide range of the linear velocity can be covered simply by changing
the reference data clock period with the same pulse strategy i.e. without
changing the pulse strategy, or the least changing the pulse strategy.
In order to solve these problems, the present inventors have proposed to
use a reflective layer having a specific film thickness and volume
resistivity, particularly to use a reflective layer mainly comprising
silver or gold (see U.S. patent application Ser. No. 09/048,042).
It is preferable to use a reflective layer comprising a material such as
silver which is a metal having a high reflectance and a high thermal
conductivity since it assures an optical interference effect and also
assures a heat dissipation effect. Particularly, silver is the metal of
the highest thermal conductivity and, therefore, relatively thin silver
layer of less than 100 nm thickness has enough heat dissipating effect.
Moreover, silver is preferable since it is easily formed into a film and
it is preferable from economical viewpoint.
However, the present inventors have further studied and found that gold and
silver do not have a satisfactory adhesion to a dielectric material, and
also that since silver is corroded with sulfur, a protective layer
containing sulfur causes a problem.
Also, the present inventors have proposed a multi-layered reflective layer
comprising a first reflective layer comprising aluminum alloy provided on
a protective layer and a second reflective layer comprising silver
provided thereon in the above-mentioned U.S. patent application. However,
in such a case, it has been found that mutual atomic diffusion between
aluminum and silver is caused and consequently that a storage stability is
poor and recording can not properly be made when stored at a high
temperature under a high humidity although a satisfactory recording
property can be obtained immediately after forming a film.
That is, when using a material comprising silver as the main component is
used as a reflective layer, a storage stability is unsatisfactory, and a
recording sensitivity, a recording signal intensity or the like is changed
when recording is carried out after storing a medium under a severe
environment for a long term.
The present invention has been made for solving the above-mentioned
problems, and an object of the present invention is to provide an optical
information recording medium having satisfactory disk properties in a wide
linear velocity range and in a wide irradiation power range. Also, another
object of the present invention is to provide an optical information
recording medium excellent in storage stability.
The essential features of the present invention reside in an optical
information recording medium comprising a substrate, a recording layer, a
protective layer containing a sulfur atom, an intermediate layer in
contact with the protective layer and a reflective layer containing silver
as the main component in contact with the intermediate layer, wherein the
intermediate layer comprises an element which does not form a compound
with silver, the element contained in the intermediate layer having a
solid solubility of at most 5 atomic % to silver and silver having a solid
solubility of at most 5 atomic % to the element contained in the
intermediate layer, on the side in contact with the reflective layer, and
the intermediate layer comprises an element less reactive to sulfur or its
sulfide comprises chemically stable elements, on the side in contact with
the protective layer, and further reside in an optical information
recording medium comprising a substrate, a recording layer, a protective
layer containing a sulfur atom, an intermediate layer in contact with the
protective layer and a reflective layer containing silver as the main
component in contact with the intermediate layer, wherein the intermediate
layer comprises an element which forms a continuous series of solid
solutions with silver, and still further reside in an optical information
recording medium comprising a substrate, a recording layer, a protective
layer containing a sulfur atom, an intermediate layer in contact with the
protective layer and a reflective layer containing silver as the main
component in contact with the intermediate layer, wherein the intermediate
layer comprises amorphous carbon or an oxide, a nitride or a carbide of a
semiconductor or metal.
FIG. 1 illustrates an example of an optical information recording medium of
the present invention, wherein 1 is a substrate, 2 is a lower protective
layer, 3 is a phase-change type recording layer, 4 is an upper protective
layer, 5 is a first reflective layer, 6 is a diffusion-preventing layer, 7
is a second reflective layer and 8 is a protective coating.
FIG. 2 illustrates an example of recording pulse strategy used in the
present invention.
FIG. 3 illustrates a heat-diffusion state of a recording layer, wherein 1
is a substrate, 2 is a lower protective layer, 3 is a phase-change type
recording layer, 4 is an upper protective layer, and 9 is a reflective
layer.
FIG. 4 is a graph illustrating a recording power dependency of jitter at
2.4 m/s in Example 1.
FIG. 5 is a graph illustrating a recording power dependency of jitter at
4.8 m/s in Example 1.
FIG. 6 illustrates another example of recording pulse strategy used in the
present invention.
FIG. 7 is a graph illustrating a recording power dependency of jitter,
reflectivity and modulation in Example 5.
FIG. 8 is a graph illustrating a recording power dependency of jitter in
Example 6.
FIG. 9 is a graph illustrating a recording power dependency of reflectivity
and modulation in Example 6.
The present inventors have found that a medium using a combination of a
protective layer containing a sulfur atom and a reflective layer
containing silver as the main component is remarkably deteriorated after
repetitive overwriting or storing for a long time although its initial
properties are satisfactory, and consequently that it is hard to
practically use as it is.
Also, the present inventors have found that if a layer containing aluminum
as the main component is provided as an intermediate layer between a
protective layer containing a sulfur atom and a reflective layer
containing a silver as the main component, its storage stability is poor
and it is remarkably deteriorated by environmental resistance test
although its initial properties are satisfactory, and consequently it is
not suitable for practical use.
Accordingly, the present inventors have variously studied with regard to an
intermediate layer provided between a protective layer containing a sulfur
atom and a reflective layer containing silver as the main component, and
have discovered that a practical medium excellent in storage stability and
repetitive recording properties can be provided when the intermediate
layer satisfies specific requirements. The present invention has been
accomplished on the basis of this discovery.
In the present invention, the intermediate layer comprises an element which
does not form a compound with silver, the element contained in the
intermediate layer having a solid solubility of at most 5 atomic % to
silver and silver having a solid solubility of at most 5 atomic % to the
element contained in the intermediate layer, on the side in contact with
the reflective layer, and the intermediate layer comprises an element less
reactive to sulfur or its sulfide comprises chemically stable elements on
the side in contact with the protective layer. Alternatively, the
intermediate layer comprises an element which forms a continuous series of
solid solutions with silver, or comprises amorphous carbon or an oxide, a
nitride or a carbide of a semiconductor or metal.
East Germany Patent Number 98782 discloses a magneto-optical recording
medium having a ZnS layer, a ferromagnetic MnBi layer, a ZnS layer and a
silver layer laminated on a glass substrate, but discloses nothing about
the above-mentioned problems concerning repetitive overwriting properties
and storage stability caused by the combination use of a protective layer
containing a sulfur atom and a reflective layer containing silver as the
main component, and does not disclose nor suggests that the
above-mentioned problems can be solved by providing an intermediate layer
satisfying the above-mentioned specific requirements.
JP-A-8-329525 discloses a phase-change type recording medium having an
Au.sub.50 Ag.sub.50 reflective layer, a (ZnS).sub.80 (SiO.sub.2).sub.20
protective layer, a (Ge.sub.2 Sb.sub.2 Te.sub.5).sub.90 (Cr.sub.4
Te.sub.5).sub.10 recording layer, a (ZnS).sub.80 (SiO.sub.2).sub.20
protective layer, a Si first reflective layer and an Al.sub.97 Ti.sub.3
second reflective layer laminated on a polycarbonate substrate, and Al,
Au, Cu, Pt, Pd, Sb--Bi and their alloys, together with Ag and its alloy,
are illustrated as the first/second reflective layer materials. However,
this reference discloses nothing about the problem concerning repetitive
overwriting properties and storage stability caused by the combination use
of a protective layer containing a sulfur atom and a reflective layer
containing silver as the main component, and does not disclose nor
suggests to solve the above-mentioned problems by providing an
intermediate layer satisfying the above-mentioned specific requirements.
JP-A-9-185846 discloses a phase-change type recording medium having a
(ZnS).sub.80 (SiO.sub.2 ).sub.20 protective layer, a (Cr.sub.4
Te.sub.5).sub.7 (Ge.sub.2 Sb.sub.2 Te.sub.5).sub.93 recording layer, a
(ZnS).sub.80 (SiO.sub.2).sub.20 protective layer, a Si first reflective
layer, a tungsten diffusion-preventing layer and an Al.sub.97 Ti.sub.3
second reflective layer laminated on a polycarbonate substrate, and Al,
Au, Cu, Mo, Ta, W, Co, Pt and their alloys, together with Ag and its
alloy, are illustrated as the first/second reflective layer materials, but
this is merely one example among many elements. Thus, this reference does
not disclose to use a material containing silver as the main component for
a reflective layer and its utility. Further, as a matter of fact, this
reference discloses nothing about the problem concerning repetitive
recording properties and storage stability caused by the combination use
of a protective layer containing a sulfur atom and a reflective layer
containing silver as the main component, and does not disclose nor
suggests that the problems can be solved by providing an intermediate
layer satisfying the above-mentioned specific requirements.
The optical information recording medium of the present invention can be
used as a medium for various recording systems such as a magneto-optical
recording medium, a phase-change type recording medium and the like, but
is used preferably for a phase-change type recording medium, particularly
a phase-change type recording medium utilizing a reflectance difference
between a crystalline state and an amorphous state.
Hereinafter, the present invention is described with regard to a preferable
phase-change type recording medium in more details.
Most of conventional phase-change type optical disks have a protective
layer, a recording layer, a protective layer and a reflective layer
provided on a substrate in this order mainly by sputtering method, and a
UV ray-curable resin layer is further provided thereon. The reflective
layer is provided for the purposes of positively utilizing an optical
interference effect to enlarge a signal amplitude and also working as a
heat-dissipation layer, and in the case of a phase-change type recording
medium, the reflective layer has a function of producing a super-cooled
state necessary for the formation of amorphous marks. For this purposes, a
metal having a high reflectance and a high thermal conductivity is
generally used as the reflective layer, examples of which include Au, Ag,
Al and the like.
In view of economical aspect and easy film-formation, Ag is preferable. Ag
is relatively cheap as a sputtering target, and provides a stable
discharge, a high film-forming speed and a high stability in air and also
provides excellent properties in respect of reflectance and thermal
conductivity. Also, the same effect can be expected with regard to a
system having a small amount of impurities mixed with Ag. However, some
films provided in contact with Ag have a bad interference to Ag. For
example, when a layer containing such an element as to be easily diffused
into the Ag film is provided in contact with Ag, a heat conductivity is
largely reduced by undesirable alloying. In such a case, when a signal is
newly recorded, due to a heat distribution difference at the time of
recording, a recording sensitivity is changed, or signal properties
becomes worse since marks are not clearly formed. These phenomena are not
preferable.
Thus, in the case of a conventional phase-change optical disk, it is
general to use an Al alloy having Ta, Ti, Cr, Mo, Mg, Zr, V, Nb or the
like added in an amount of from 0.5 to 5 atomic % as a reflective layer
without using Ag. However, even in such a case, the same phenomena as
mentioned above occur after storing under severe environment, since such
an Al alloy probably causes a segregation of compounds of Al and additive
element, resulting in a change in reflectivity and thermal conductivity.
When the recording layer comprises an alloy thin film of Ma.sub.w (Sb.sub.z
Te.sub.1-z).sub.1-w wherein 0.ltoreq.w.ltoreq.0.3, 0.5.ltoreq.z.ltoreq.0.9
and Ma is at least one component selected from the group consisting of In,
Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Pb, Cr, Co, O, N, S, Se, Ta, Nb,
V, Bi, Zr, Ti, Mn, Mo, Rh and a rare earth element), mark shapes are
likely to be influenced by a difference in heat distribution.
Examples of an element easily diffusable into Ag include Al, S and the
like. Accordingly, in the case of using ZnS--SiO.sub.2 often usable for a
phase-change type optical disk as a protective layer, when an Ag
reflective layer is provided in contact with the ZnS--SiO.sub.2 protective
layer in the same manner as in the above conventional type structure,
recording properties are changed as a lapse of time due to a corrosion of
Ag layer. Also, many defects are observed by observation with a microscope
on the Ag side, and even in a case of a new disk, recording properties are
sometimes deteriorated by repeated overwriting about 100 to 1000 times,
and the disk becomes useless.
Therefore, the present inventors have proposed to provide an intermediate
layer which prevents diffusion from a protective layer to a reflective
layer containing Ag as the main component which does not cause a practical
damage on heat conductivity.
The phase-change type recording medium of the present invention comprises a
substrate, a phase-change type recording layer, a protective layer and a
reflective layer, and an intermediate layer is provided in contact with
the protective layer and the reflective layer. For example, a lower
protective layer, a phase-change type recording layer, an upper protective
layer, an intermediate layer and a reflective layer are provided on a
substrate in this order, or a reflective layer, an intermediate layer, a
protective layer, a phase-change type recording layer and a protective
layer are provided on a substrate in this order. The above multi-layered
structure may be provided on both sides of the substrate.
When the intermediate layer is composed of a specific metal or alloy, the
intermediate layer comprises an element which does not form a compound
with silver, the element contained in the intermediate layer having a
solid solubility of at most 5 atomic % to silver and silver having a solid
solubility of at most 5 atomic % to the element contained in the
intermediate layer, on the side in contact with the reflective layer, and
the intermediate layer comprises an element less reactive to sulfur or its
sulfide comprises chemically stable elements on the side in contact with
the protective layer.
Thus, the intermediate layer must comprise an element satisfying the
above-mentioned specific conditions, and the conditions are determined
basically by referring to a binary alloy phase diagram of the element
contained in the intermediate layer with silver or sulfur. Such a binary
alloy phase diagram is described in "Constitution of Binary Alloys", (Max
Hansen and Kurt Anderko, second edition (1985), Genium Publishing
Corporation, New York).
Silver is easily diffused into other metal even at a low temperature of
less than 100.degree. C., and the formation of a solid solution or a
compound due to such a diffusion is not preferable since inherent high
reflectivity or high thermal conductivity of silver is degraded.
Therefore, the intermediate layer must comprise an element which does not
form a solid solution or a compound with silver on the side in contact
with the reflective layer.
In the present invention, the condition "an element does not form a solid
solution with silver" means that the element does not form a solid
solution with silver at all and that both of a solid solubility of the
element to silver and a solid solubility of silver to the element are at
most 5 atomic % and the element is quite hardly soluble to silver.
The term "solid solubility" refers to the maximum solid solubility in the
total temperature range as far as it is a solid state. In the binary alloy
phase diagram according to the above Hansen's reference, examples of the
element which does not form a solid solution with silver nor forms a
compound with silver, include sodium, lead, bismuth, silicon, tantalum,
cobalt, chromium, tungsten, vanadium, and the like.
Among them, sodium, lead and bismuth have an eutectic temperature in a
relatively low temperature range of at most 500.degree. C., and are
therefore relatively thermally unstable. That is, according to the phase
diagram, respective eutectic temperatures are sodium (97.degree. C.), lead
(304.degree. C.), bismuth (262.degree. C.), silicon (830.degree. C.) and
chromium (961.degree. C.).
Detailed phase diagrams are not described with regard to tantalum, cobalt,
tungsten and vanadium, but it is known that these elements do not form a
compound with silver and that they are substantially insoluble to silver
even in a melted state. Nickel has almost no solid solubility to silver,
and silver has a slight solid solubility to nickel but its solid
solubility is believed much lower than 5 atomic %.
Sodium is not preferable since it is unstable in the atmosphere.
On the other hand, zirconium, magnesium, manganese, indium, titanium,
antimony, germanium, tellurium, zinc and the like are not suitable since
they form a compound with silver or form a solid solution in a relatively
high concentration range. Further, aluminum also forms a solid solution
with silver, and it is therefore not preferable as an element for the
intermediate layer on the side in contact with silver.
The stability at the interface of a laminated layer of silver and an
element (silicon, tantalum, cobalt, chromium, tungsten or vanadium)
considered to be preferable according to the above consideration, was
confirmed actually by applying an acceleration test under high temperature
and humidity to the laminated thin film, and it was also confirmed by
measuring a thermal conductivity change by alloy-formation that a heat
dissipation effect was not lowered.
Among these elements, tantalum and nickel are most preferable elements
since they hardly cause separation due to an internal stress in the film.
However, even an element forming a solid solution with silver is sometimes
preferable in the case of forming a continuous series of solid solutions
with silver since it does not cause a phase separation and it does not
unfavorably affect on a thermal conductivity. Preferable examples of
forming a continuous series of solid solutions with silver include gold
and palladium.
On the other hand, the intermediate layer comprises an element less
reactive with a highly corrosive sulfur, i.e. an element which does not
form a compound with sulfur at all in the phase diagram, or an element,
the sulfide of which forms a chemically stable passive state at the
interface and achieves a diffusion-preventing effect, on the side in
contact with the protective layer containing sulfur.
The stability of the sulfide is described in a literature or can be
confirmed by measuring thermogravimetric spectrum. According to the
above-mentioned Hansen's phase diagram, aluminum is a rare element which
does not form a compound with sulfur at all and is most preferable.
It is considered that silicon, tantalum, tungsten, germanium and vanadium
respectively form SiS.sub.2, TaS.sub.2, WS.sub.2, GeS.sub.2 and V.sub.2
S.sub.3, but it is a temperature higher than 500.degree. C. at which a
thermogravimetric change such as melting, decomposition or sublimation is
caused with regard to these compounds.
Cobalt and chromium form many sulfides, but according to the phase diagram,
a melting point and a decomposition temperature of these sulfides are
higher than 500.degree. C. The stability was experimentally confirmed by
forming films of these elements on a ZnS:SiO.sub.2 protective layer, and
according to the experimental results, corrosion was not caused at least
by sulfide-formation reaction and a reflectance did not change.
According to the phase diagram, sulfides of silver and copper such as
Ag.sub.2 S and Cu.sub.2 S are thermally stable, but according to the
experimental results, they exhibited some unstability on a ZnS:SiO.sub.2
protective layer. In the phase diagram, their solid phases exhibit phase
transformation at a temperature higher than 500.degree. C., and it is
considered that they are not always stable.
When the intermediate layer comprises a compound, the intermediate layer
comprises amorphous carbon or an oxide, a nitride or a carbide of a
semiconductor or metal. They are stable compounds. It is preferable that
they are heat resistant compounds having a melting point of at least
1000.degree. C. when a recording layer is a phase change medium.
A sulfide is not preferable as an intermediate layer since it is reactive
with silver.
The compound intermediate layer preferably comprises a compound transparent
to the wavelength of a light source used for light recording or retrieving
since inherent high reflectance of silver can be effectively utilized. In
this respect, the amorphous carbon is preferably hydrogenated amorphous
carbon having a high transparency.
The reflective layer contains silver as the main component, and this means
that the reflective layer contains silver in an amount of at least 70
atomic %. Preferably, the reflective layer contains silver in an amount of
at least 95 atomic %, more preferably at least 98 atomic %. Particularly,
the reflective layer comprises pure silver or a silver alloy containing at
least one component selected from the group consisting of Ti, V, Ta, Nb,
W, Co, Cr, Si, Ge, Sn, Sc, Hf, Pd, Rh, Au, Pt, Mg, Zr, Mo and Mn in an
amount of from 0.2 atomic % to 2 atomic %.
Among them, Pd, Mg, and Ti are most preferable.
On the other hand, the protective layer, especially on the recording layer,
contains sulfur, and preferably contains a sulfide such as zinc sulfide,
tantalum sulfide and a rare earth sulfide.
Specifically, the material is preferably a composite dielectric which
contains above described materials alone or its mixture in an amount of
from 20 to 90 mol % and one or more heat resistant compounds having a
melting point or a decomposition temperature of at least 1000.degree. C.,
including an oxide, a nitride, a fluoride or a carbide of a metal or a
semiconductor.
The intermediate layer has a thickness of generally at least 10 .ANG.,
preferably at least 50 .ANG., but generally at most 1000 .ANG. to make
full use of the high heat conductivity of silver reflective layer,
preferably at most 500 .ANG., more preferably at most 200 .ANG.. The
intermediate layer comprises one or more layers. Among them, preferable
examples of the intermediate layer include the following two embodiments.
Embodiment (1)
Intermediate layer comprising two layers of a layer containing aluminum as
the main component and a layer preventing alloy-formation between aluminum
and silver, in which the former layer is in contact with a protective
layer and the latter layer is in contact with a reflective layer.
Embodiment (2)
Intermediate layer comprises an element which does not form a compound with
silver, the element contained in the intermediate layer having a solid
solubility of at most 5 atomic % to silver and silver having a solid
solubility of at most 5 atomic % to the element contained in the
intermediate layer, on the side in contact with the reflective layer, and
the element is less reactive to sulfur or its sulfide comprises chemically
stable elements on the side in contact with the protective layer.
Alternatively, the intermediate layer comprises an element which forms a
continuous series of solid solutions with silver, or comprises amorphous
carbon or an oxide, a nitride or a carbide of a semiconductor or metal.
The Embodiment (1) is described in more details hereinafter. The Embodiment
(1) is expressed generally in the following manner. A first reflective
layer, a diffusion-preventing layer and a second reflective layer having a
volume resistivity of from 20 n.OMEGA..multidot.m to 80
n.OMEGA..multidot.m are provided on a protective layer. In this case, the
layer containing aluminum as the main component corresponds to the first
reflective layer, and the layer of preventing alloy-formation between
aluminum and silver corresponds to the diffusion-preventing layer, and the
reflective layer containing silver as the component corresponds to the
second reflective layer.
The reason why aluminum forming a solid solution with silver is used by
providing a diffusion-preventing layer, is described below. An object of
the present invention is to provide a stable optical information recording
medium by effectively utilizing the inherent high thermal conductivity of
silver, but it is preferable for this object that an intermediate layer
provided between a protective layer and silver has a high thermal
conductivity. Aluminum itself has an inherent high reflectance, and
achieves a preferable function as a reflective layer having a high
reflectance and a high thermal conductivity to the total intermediate
layer and silver reflective layer. Aluminum is the most preferable
material excellent in respects of reflectance, thermal conductivity and
chemical stability, except for being reactive to silver, and is therefore
worthy to be used even by providing a diffusion-preventing layer.
According to this structure, even when an intermediate layer is provided
between a protective layer and a reflective layer, a thermal conductivity
is not substantially hindered, and a medium having satisfactory disk
properties in a wide linear velocity range and in a wide power range and
having an excellent storage stability can be obtained, and this is quite
preferable for practical use.
Hereinafter, with regard to the Embodiment (1), an explanation is made
based on this generalized embodiment.
As shown in FIG. 1, a medium of the Embodiment (1) generally has a
structure of substrate 1/lower protective layer 2/phase-change recording
layer 3/upper protective layer 4/first reflective layer
5/diffusion-preventing layer 6/second reflective layer 7. Further, it is
preferable to have a protective coating layer 8 of UV ray-curable or
heat-curable resin coated thereon.
In the Embodiment (1), the material of the phase-change recording layer 3
may be well known conventional ones such as GeSbTe, InSbTe, AgSbTe,
AgInSbTe, AgGeSbTe or the like, but preferably an alloy comprising SbTe
alloy in the vicinity of Sb.sub.70 Te.sub.30 eutectic point as the main
component, which is stable in either crystalline or amorphous state and is
capable of causing a rapid phase-change between the two states. This
material is the most practical material which hardly causes segregation
when carrying out repetitive overwriting. Specifically, Ma.sub.w (Sb.sub.z
Te.sub.1-z).sub.1-w alloy (wherein 0.ltoreq.w.ltoreq.0.3,
0.5.ltoreq.z.ltoreq.0.9 and Ma is at least one component selected from the
group consisting of In, Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Pb, Cr,
Co, O, N, S, Se, Ta, Nb, V, Bi, Zr, Ti, Mn, Mo, Rh and a rare earth
element) is preferable, and more preferably 0.ltoreq.w.ltoreq.0.2 and
0.6.ltoreq.z.ltoreq.0 8.
According to the study of the present inventors, a linear velocity
dependency is determined by Sb and Te as the main components, and in the
vicinity of Sb.sub.70 Te.sub.30 eutectic point, a crystallization speed
tends to become higher as Sb/Te ratio becomes larger.
A ternary system material having Ge or In added in the vicinity of the
eutectic composition is less deteriorated in respect to repetitive
overwriting by a specific recording pulse pattern as compared with
conventionally well known GeTe--Sb.sub.2 Te.sub.3, InTe--Sb.sub.2 Te.sub.3
pseudo-binary alloy materials, and jitter of mark edge in the case of mark
length-recording is small, thus being an excellent material. Further, this
material can assure a high crystallization temperature of more than
150.degree. C. and is excellent in archival stability.
This recording layer is usually amorphous in the state immediately after
film-formation, and it is therefore preferable to crystallize the whole
recording layer surface, thereby making it an initialized state
(unrecorded state), as mentioned below.
The Embodiment (1) provides a medium capable of a satisfactory overwriting
in a wide linear velocity range, the minimum linear velocity/the maximum
linear velocity ratio being at lest two times during overwriting.
More concretely, there is provided a medium capable of a satisfactory
overwriting in the range of from 2 times (2.4-2.8 m/s) to 4 times (4.8
m/s-5.6 m/s) of CD linear velocity or of from 1 time (3.5 m/s) to 2 times
(7 m/s) of DVD linear velocity.
For this reason, the recording layer composition must have such a high
recrystallization ability as to fully erase at about 10 m/s.
As mentioned above, the recording layer of the Embodiment (1) preferably
has Sb.sub.70 Te.sub.30 eutectic composition as a base, and a linear
velocity dependency is influenced by a Sb/Te ratio. Thus, the above
recording layer composition is preferably Ma.sub.w (Sb.sub.z
Te.sub.1-z).sub.1-w alloy (wherein 0.ltoreq.w.ltoreq.0.3,
0.5.ltoreq.z.ltoreq.0.9, and Ma is at least one component selected from
the group consisting of In, Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Pb,
Cr, Co, O, N, S, Se, Ta, Nb, V, Bi, Zr, Ti, Mn, Mo, Rh and a rare earth
element).
More concretely, a preferably example includes Mb.sub..alpha.1
In.sub..beta.1 Sb.sub..gamma.1 Te.sub..eta.1 composition (wherein
0.03.ltoreq..alpha.1.ltoreq.0.1, 0.03.ltoreq..beta.1.ltoreq.0.08,
0.55.ltoreq..gamma.1.ltoreq.0.65, 0.25.ltoreq..eta.1.ltoreq.0.35,
0.06.ltoreq..alpha.1+.beta.1.ltoreq.0.13,
.alpha.1+.beta.1+.gamma.1+.eta.1=1, and Mb is at least one of Ag and Zn).
A more preferable example includes a composition satisfying
0.03.ltoreq..alpha.1.ltoreq.0.1, 0.05.ltoreq..beta.1.ltoreq.0.08,
0.6.ltoreq..gamma.1.ltoreq.0.65, 0.25.ltoreq..eta.1.ltoreq.0.30,
0.06.ltoreq..alpha.1+.beta.1.ltoreq.0.13 and
.alpha.1+.beta.1+.gamma.1+.eta.1=1 in the above composition.
In this composition range, a satisfactory erasability of more than 25 dB
can be obtained at the time of overwriting at up to 10 m/s. Also, a
composition excellent in archival stability can be provided.
In achieves an effect of raising a crystallization temperature and
improving an archival stability, and it is preferable to add In in an
amount of at least 3 atomic % in order to secure a storage stability at
room temperature, but if the amount of In exceeds 8 atomic %, a phase
separation tends to occur and segregation tends to occur by repetitive
overwriting. Thus, a more referable amount of In is from 5 atomic % to 8
atomic %.
Ag or Zn facilitates initialization of an amorphous film immediately after
film-formation. An amount of at most 10 atomic % is almost sufficient for
initialization, and if its amount is too large, an archival stability is
deteriorated.
Also, if a total amount of Ag or Zn and In exceeds 13 atomic %, a
segregation is unpreferably likely to occur at the time of repetitive
overwriting.
A preferable other example of the recording layer includes a composition of
Mc.sub.v Ge.sub.y (Sb.sub.x Te.sub.1-x).sub.1-y-v (wherein
0.6.ltoreq.x.ltoreq.0.8, 0.01.ltoreq.y.ltoreq.0.15,
0.ltoreq.v.ltoreq.0.15, 0.02.ltoreq.y+v.ltoreq.0.2 and Mc is at least one
of Ag and Zn).
According to this composition, acceleration of precipitation of a low
melting metal In and an In-modified alloy in the above MbInSbTe alloy can
be improved.
However, on the other hand, initialization process in manufacturing takes
unpreferably a long time by the addition of Ge. This unpreferable
phenomenon is caused rapidly by the addition of Ge.
In order to improve precipitation of In and to improve retardation of
initialization by the addition of Ge, it is preferable to use a
composition of Md.sub..alpha.2 In.sub..beta.2 Ge.sub..delta.2
Sb.sub..gamma.2 Te.sub..eta.2 (wherein 0.01.ltoreq..alpha.2.ltoreq.0.1,
0.001.ltoreq..beta.2.ltoreq.0.1, 0.01.ltoreq..beta.2.ltoreq.0.1,
0.5.ltoreq..gamma.2.ltoreq.0.7, 0.25.ltoreq..eta.2.ltoreq.0.4,
0.03.ltoreq..beta.2+.delta.2.ltoreq.0.15,
.alpha.2+.beta.2+.delta.2+.gamma.2+.eta.2=1 and Md is at least one of Ag
and Zn).
Generally, a thickness of the phase-change recording layer 3 is preferably
in the range of from 10 nm to 100 nm.
If the thickness is smaller than 10 nm, a satisfactory optical contrast is
hardly obtainable, and a crystallization speed tends to become low, and
erasing in a short time tends to become hard. On the other hand, if the
thickness exceeds 100 nm, an optical contrast is also hardly obtainable,
and a crack tends to occur.
Particularly, in order to obtain such a satisfactory contrast as to provide
a retrieving compatibility with CD or DVD, it is more preferable to use a
thickness of from 10 nm to 30 nm. If the thickness is less than 10 nm, a
reflectivity becomes too low, and if the thickness exceeds 30 nm, a heat
capacity becomes large, and a recording sensitivity tends to become poor.
Plastic deformation by heat cycle during repetitive overwriting tends to be
more accumulated as the recording layer is made thicker. From this point
of view, it is preferable to use a thickness of at most 30 nm, more
preferably at most 25 nm.
As mentioned above, it is preferable to use such a composition of the
recording layer as to be suitable for high speed overwriting. If such a
composition as to be capable of sufficiently erasable at a high linear
velocity is used, the recording layer once melted is easily recrystallized
when recording at a low linear velocity, and therefore a satisfactory
amorphous mark is hardly formed. Especially to solve this problem, the
present invention using a silver based reflective layer is useful.
In conventional GeTe--Sb.sub.2 Te.sub.3 system, in order to obtain a
satisfactory cooling speed of recording layer at a low linear velocity,
"rapid cooling structure" wherein the film thickness of upper protective
layer 4 is thin, is preferable, and accordingly the film thickness is
generally made from 20 nm to 30 nm.
This tendency is expressed, for example, in symposium on phase-change
optical recording held every year since 1991 (see the text published by
the Society of Applied Physics, the society for the study of phase-change
optical recording).
The main reason is to effectively work heat dissipation to a reflective
layer.
The "rapid cooling structure" employs such a layer structure as to enhance
heat dissipation and to increase a cooling speed at the time of
recrystallization of molten recording layer, thereby solving trade-off
between recrystallization during amorphous mark formation and a high
erasability by high speed crystallization.
Thus, if the film thickness of upper protective layer 4 is too large, it
takes a long time until the heat of recording layer 3 reaches the
reflective layer, and the heat dissipation effect by the reflective layer
does not work effectively.
The present inventors have found that the linear velocity dependency can be
more improved than conventional rapid cooling structure with aluminum
alloy reflective layer by combining a reflective film having a high
thermal conductivity with the upper protective layer 4 having rather a
large thickness of from 30 nm to 60 nm, like silver, more preferably from
35 nm to 55 nm.
This is explained in more details with reference to FIG. 3.
First, it is necessary for recording to raise a temperature of recording
layer higher than a melting point, but since it requires a certain
retention time for thermal conduction, in the temperature-rising process
(initial several tens nanoseconds or less), thermal conduction is not
noticeable in the direction of horizontal plane surface (transverse
direction) and temperature distribution is determined almost by thermal
conduction in the direction of film thickness (FIG. 3(a)).
Accordingly, the thermal conduction in the film thickness direction
effectively works for raising the temperature of the leading-edge of the
recording mark to a predetermined temperature.
On the other hand, after several tens nanoseconds from the initiation of
temperature-rising, the thermal conduction in the horizontal plane surface
direction becomes important for temperature distribution as illustrated in
FIG. 3(b).
This is because the thermal conduction in the film thickness direction is
heat diffusion between a distance of at most 0.1 .mu.m, while the thermal
conduction in the plane surface direction is heat diffusion between a
distance of 1 .mu.m order.
Particularly, a cooling speed of recording layer controlling
amorphous-forming process depends on this horizontal plane surface
temperature distribution, and the linear velocity dependency of the
cooling speed is strictly controlled by the horizontal plane temperature
distribution.
At a low linear velocity, since a scanning speed of light beam is low, heat
relatively widely reaches the periphery part within the same irradiation
time, and the thermal conduction in the horizontal plane surface direction
is influential.
Also, at the end part (trailing-edge) of a long mark continuously
irradiated with a recording light beam for a relatively long time, the
thermal conduction in the horizontal plane surface direction is
influential.
Accordingly, in order to satisfactorily conduct mark length recording in
such a wide linear velocity range as to make the maximum velocity/the
minimum velocity ratio at least 2 times during overwriting, it is
necessary to accurately control not only the temperature distribution in
the film thickness direction and its time change but also the temperature
distribution in the horizontal plane surface direction and its time
dependency.
In FIG. 3(b), by making a thermal conductivity of the upper protective
layer low and making the upper protective layer so as to have an
appropriate thickness, a certain retardation effect can be achieved on
heat flowing to a reflective layer and the temperature distribution in the
horizontal plane surface direction can be properly controlled.
The conventional "rapid cooling structure" does not always consider the
retardation effect of thermal conduction.
In the present invention, in order to fully achieve the retardation effect
of thermal conduction, a material having a low thermal conductivity is
preferably used for the upper protective layer 4, and preferable examples
include a material containing ZnS, ZnO, TaS.sub.2 or a rare earth sulfide
alone or its mixture in an amount of from 20 mol % to 90 mol %. Further, a
composite dielectric material containing a heat resistant compound having
a melting point or a decomposition temperature of at least 1000.degree. C.
is preferable.
More preferable examples include a composite dielectric material containing
a sulfide of a rare earth material such as La, Ce, Nd or Y in an amount of
from 50 mol % to 90 mol %, and a composite dielectric material containing
ZnS, ZnO or a rare earth sulfide in an amount of from 70 to 90 mol %.
Examples of the heat resistant compound material having a melting point or
a decomposition point of at least 1000.degree. C. include an oxide, a
nitride or a carbide of Mg, Ca, Sr, Y, La, Ce, Ho, Er, Yb, Ti, Zr, Hf, V,
Nb, Ta, Zn, Al, Si, Ge, Pb or the like, or a fluoride of Ca, Mg, Li or the
like.
Particularly, examples of a material to be mixed with ZnO include a sulfide
of a rare earth element such as Y, La, Ce, Nd or the like, or a mixture of
sulfide and oxide.
A thin film containing SiO.sub.2, Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3, AlN,
SiN or the like as the main component is not preferable since its thermal
conductivity is too high. Particularly, the upper protective layer
preferably contains sulfur, and more preferably contains a sulfide such as
ZnS, TaS.sub.2, a rare earth sulfide or the like.
In view of mechanical strength, it is preferable that these protective
layers have a film density of at least 80% of bulk state.
In a case where a thin film of a dielectric mixture is employed, a
theoretical density of the following formula is used as the bulk density:
.rho.=.SIGMA.m.sub.i.rho..sub.i (1)
m.sub.i : mol concentration of each component i
.rho..sub.i : bulk density of each component
The upper protective layer 4 also has an effect of preventing mutual
diffusion between a recording layer 3 and a reflective layer 5.
As mentioned above, the upper protective layer 4 has a film thickness of
generally from 30 nm to 60 nm, preferably from 35 nm to 55 nm.
If the film thickness is less than 30 nm, a satisfactory retardation effect
of thermal conduction can not be achieved, and if the film thickness
exceeds 60 nm, a satisfactory heat dissipation effect to the reflective
layer can not be achieved, and plastic deformation by heat cycle during
repetitive overwriting is accumulated within the inside of the protective
layer, and deterioration tends to be accelerated in proportion to the
number of overwriting.
In the Embodiment (1), when the upper protective layer 4 is simply made
thick in order to achieve the retardation effect of thermal conduction by
the above protective layer, a cooling rate becomes too low, and it is
therefore necessary to use a reflective layer having such a high thermal
conductivity as to achieve a satisfactory rapid cooling effect after a
predetermined retardation time.
However, it is pretty difficult to measure the thermal conductivity of such
a thin film as the reflective layer employed in the present invention, and
its reproducibility is likely to become poor.
Generally, the thermal conductivity of a thin film is small and is largely
different from the thermal conductivity of bulk state. Particularly, a
thin film of at most 40 nm is not preferable since its thermal
conductivity becomes smaller in the order of one figure by the influence
of island structure of the initial growing stage.
Therefore, in the present invention, an electric resistance of the
reflective film is made a standard indication in place of the thermal
conductivity.
With regard to a material like a metal wherein electron flow and scattering
process mainly controls thermal or electric conduction, there is a
satisfactory proportional relation between a thermal conductivity and an
electric conductivity, and therefore, it is possible to estimate a thermal
conductivity by measuring an electric resistance.
The electric resistance of a thin film is expressed by a resistivity value
defined by its film thickness and an area of measured zone. For example, a
volume resistivity and a sheet resistivity are measured generally by four
probe resistivity method as defined by JIS K7194.
By this method, a satisfactory data having better reproducibility can be
obtained more conveniently than measuring a thermal conductivity of a thin
film. As the volume resistivity is lower, the thermal conductivity becomes
higher proportionally.
In the Embodiment (1), the reflective layer is composed of at least two
layers including a first reflective layer containing aluminum as the main
component and having a film thickness of from 5 nm to 50 nm and a second
reflective layer containing silver as the main component and having a film
thickness of from 40 nm to 200 nm and a volume resistivity of from 20
n.OMEGA..multidot.m to 80 n.OMEGA..multidot.m.
That is, at least one layer works as the above-mentioned low volume
resistivity material which substantially achieves a heat dissipation
effect, and the other layer works so as to improve corrosion resistance,
adhesion to a protective layer and hillock resistance.
Particularly, when the upper protective layer contains a sulfide, it is
preferable to make such a structure since the sulfide is corrosive to a
metal such as Ag.
The first reflective layer preferably has a thickness of from 5 nm to 50
nm. If the thickness is less than 5 nm, a protective effect is
insufficient, and if the thickness exceeds 50 nm, a heat dissipation to
the second reflective layer is likely to become poor.
The second reflective layer preferably has a thickness of from 40 nm to 200
nm. If the thickness is less than 40 nm, a heat dissipation effect is
likely to become insufficient. On the other hand, if the thickness exceeds
200 nm, a crack tends to occur and a film-forming time becomes long,
thereby lowering a productivity.
In order to improve optical properties, a third reflective layer may be
provided further on the second reflective layer. In such a case, the third
reflective layer may be a material having a high volume resistance.
Generally, it is possible to use a reflective layer having a high thermal
conductivity, provided that a volume resistivity is from 20
n.OMEGA..multidot.m to 150 n.OMEGA..multidot.m.
A material having a volume resistivity of less than 20 n.OMEGA..multidot.m
is hardly obtainable in a thin film state.
When the volume resistivity is higher than 150 n.OMEGA..multidot.m, a sheet
resistivity can not be lowered unless a thick film of exceeding 300 nm is
provided. According to the study by the present inventors, a material
having such a high volume resistivity does not achieve a satisfactory heat
dissipation effect even when a sheet resistivity is lowered. This is
because a heat capacity per unit area increases in the case of a thick
film.
Further, it takes a long time to make such a thick film, and it is
unpreferable also in view of production cost since a material cost
increases.
Accordingly, it is preferable to use a material having a film thickness of
at most 300 nm and a low volume resistivity (sheet resistivity of from 0.2
to 0.9.OMEGA./.quadrature.).
Thus, the first reflective layer containing aluminum as the main component
is provided to improve corrosion resistance, adhesion to a protective
layer and hillock resistance, but it should preferably have a volume
resistivity of from 20 n.OMEGA..multidot.m to 150 n.OMEGA..multidot.m
since a heat dissipation effect of a second reflective layer is hardly
achieved if a thermal conductivity is too high.
In view of adhesion and reactivity with sulfur, the first reflective layer
containing aluminum in an amount of at least 95 atomic %, more preferably
98 atomic %, is preferable. Particularly, most preferable examples include
pure aluminum or an aluminum alloy containing at least one component
selected from the group consisting of Ta, Ti, Co, Cr, Si, Sc, Hf, Pd, Pt,
Mg, Zr, Mo and Mn in an amount of from 0.2 atomic % to 2 atomic %.
Particularly, the former example is known to increase a volume resistivity
in proportion to the concentration of element added and to improve hillock
resistance (see Japan Metal Society Journal, volume 59, (1995), pp
673-678, J. Vac. Sci. Tech., A14 (1996), pp 2728-2735 and the like), and
it is employed by taking durability, volume resistivity, film-forming
speed and the like into consideration.
If the amount of the above-mentioned element to be added is less than 0.2
atomic %, a satisfactory hillock resistance is hardly obtainable although
it depends on film-forming conditions. On the other hand, if the amount of
the element exceeds 2 atomic %, the above-mentioned low resistivity is
likely to be hardly obtainable.
Particularly, in order to achieve a satisfactory archival stability, it is
preferable to add Ta or Ti, particularly Ta, as an additive.
Also, an Al--Mg--Si system alloy containing from 0.3 wt % to 0.8 wt % of Si
and from 0.3 wt % to 1.2 wt % of Mg is preferable.
In the Embodiment (1), the second reflective layer containing silver as the
main component has a volume resistivity of from 20 n.OMEGA..multidot.m to
80 n.OMEGA..multidot.m.
Preferable examples of a thin film having a volume resistivity of from 20
n.OMEGA..multidot.m to 80 n.OMEGA..multidot.m include pure silver or a
silver alloy containing at least one element selected from the group
consisting of Ti, V, Ta, Nb, W, Co, Cr, Si, Ge, Sn, Sc, Hf, Pd, Rh, Au,
Pt, Mg, Zr, Mo and Mn in an amount of from 0.2 atomic % to 2 atomic %.
Particularly, in order to improve archival stability, it is preferable to
use Ti, Pd or Mg as an additive.
A volume resistivity of the above-mentioned Al or Ag increases in
proportion to an impurity concentration.
It is considered that the addition of an impurity generally reduces a
crystal grain size and lowers a thermal conductivity by increasing
electron scattering at grain boundary. It is necessary for obtaining the
material inherently having a high thermal conductivity by enlarging a
crystal grain size to control the amount of an impurity added.
A reflective layer is formed usually by sputtering method or vacuum vapor
deposition method, but it is necessary to control the total impurity
amount at most 2 atomic %, the total impurity amount including not only an
impurity amount contained in a target or a vapor deposition material but
also water or an oxygen amount incorporated during film-formation.
For this reason, it is desirable to control a background pressure in a
process chamber to at most 1.times.10.sup.-3 Pa.
When a film is formed under a background pressure of less than
1.times.10.sup.-4 Pa, it is preferable to prevent incorporation of
impurities by controlling a film-forming rate in the range of from 1
nm/sec to 10 nm/sec.
When an impurity element is added intentionally in an amount of at least 1
atomic %, it is preferable to prevent additional incorporation of other
impurities by raising a film-forming rate to at least 10 nm/sec.
A crystal grain size is sometimes influenced by conditions such as a
sputtering pressure.
For example, an alloy film obtained by incorporating Ta in an amount of
about 2 atomic % into Al includes an amorphous phase among crystal grains,
but the ratio of a crystal phase and an amorphous phase depends on
film-forming conditions. While sputtering is carried out under a lower
pressure, the proportion of a crystalline part is increased and a volume
resistivity is lowered (thermal conductivity is raised).
Impurity composition or crystallinity in the film is influenced also by a
method of preparing an alloy target and a sputtering gas (Al, Ne, Xe and
the like) used in the sputtering process.
Accordingly, even if the above Al alloy composition is disclosed as a
reflective layer material (JP-A-3-1338, JP-A-1-169571, JP-A-1-208744 and
the like), such a volume resistivity layer structure as defined in the
present invention can not always be formed.
As mentioned above, in order to obtain a high thermal conductivity, it is
preferable to reduce an impurity amount, but in the case of pure Al or Ag
metal, corrosion resistance and hillock resistance tend to become poor,
and therefore the optimum composition is determined by taking a
satisfactory balance between the two properties into consideration.
However, it has been found that a medium having the two reflective layers
has a poor storage stability. Recording can not be carried out on such a
medium stored under conditions of a high temperature and a high humidity.
According to Auger depth profile, it has been found that the two reflective
layers are formed into an alloy.
As mentioned above, when an impurity is added, a thermal conductivity of
metal is lowered. When the alloy-formation between Al and Ag occurs, a
thermal conductivity is rapidly deteriorated, and a rapid cooling rate
necessary for forming amorphous marks can not be achieved, thereby making
the recording impossible.
Accordingly, in the Embodiment (1), the first reflective layer comprises a
material containing aluminum as the main component and the second
reflective layer comprises a material containing silver as the main
component, and consequently, it is preferable to provide a layer
preventing the diffusion of aluminum and silver as the
diffusion-preventing layer.
Examples of a material for the diffusion-preventing layer include metal,
semiconductor, metal oxide, metal nitride, metal carbide, semiconductor
oxide, semiconductor nitride, semiconductor carbide, carbon and the like.
Examples of the metal used as the diffusion-preventing layer include at
least one element selected from the group consisting of Ta, Ni, Co, Cr,
Si, W and V. Among them, Ta and/or Ni are preferable.
Examples of the oxide, nitride or carbide of metal or semiconductor used as
the diffusion-preventing layer include at least one component selected
from the group consisting of silicon oxide, silicon nitride, silicon
carbide, tantalum oxide, cerium oxide, lanthane oxide and yttrium oxide.
Specifically, examples of the amorphous carbon used as the
diffusion-preventing layer include hydrogenated amorphous carbon.
Particularly preferable examples include compounds of a material used for
the first reflective layer or the second reflective layer with oxygen
and/or nitrogen. Tantalum is also a particularly preferable material. As a
particularly preferable embodiment, a material containing aluminum as the
main component is used for the first reflective layer and a material
containing silver as the main component is used for the second reflective
layer and a material comprising tantalum or aluminum oxide is used for the
diffusion-preventing layer.
The diffusion-preventing layer can be formed by ordinary sputtering
technique, but is formed preferably by the following method.
After forming the first reflective layer, in-line vacuum is removed and the
first reflective layer is allowed to stand under the atmosphere, thereby
forming an oxide film naturally from the first reflective layer material
and oxygen (or moisture) in the atmosphere on the first reflective layer
of a disk. Thereafter, the disk is returned under vacuum, and a second
reflective layer is formed by sputtering or the like. In this manner, the
diffusion-preventing layer can be easily provided.
Also, by an ozone treatment of the surface of first reflective layer, the
diffusion-preventing layer can be provided in shorter time, because the
ozone treatment promotes a formation of passive oxide layer for preventing
diffusion.
The oxide layer of the diffusion-preventing layer thus formed prevents the
diffusion of the first reflective layer and the second reflective layer at
the boundary therebetween and disk properties provided immediately after
film-formation can be maintained as they are, even when the disk is
allowed to stand under conditions of 80.degree. C. and 85% RH for 1000
hours.
Alternatively, before or after forming the first reflective layer, an oxide
or nitride layer may be formed as a very thin sputtering film by
intentionally introducing oxygen or nitrogen into a film-forming device
under vacuum condition. This method is preferable in view of operation
efficiency since it is not necessary to break the vacuum condition.
In any case, the diffusion-preventing layer should preferably have such a
small thickness of usually at most 200 .ANG., preferably at most 100
.ANG., more preferably at most 50 .ANG. that it does not become a
hindrance as a thermal resistance. The most preferable thickness of the
diffusion-preventing layer is from 0.1 nm to 5 nm. It can be confirmed by
the same Auger depth profile that the diffusion-preventing layer
satisfactorily prevents mutual diffusion between aluminum and silver.
Hereinbefore, the present invention is described with reference to a very
limitative and concrete application example concerning CD-compatible CD-RW
of a low reflectance, but the present invention is effective to improve a
linear velocity dependency and a recording power dependency of mark length
recording of a phase-change type recording medium, and the present
invention is not specially limited to CD-RW. It is considered that the
present invention is effective also for a rewritable high density digital
video disk (DVD) presently proposed.
Also, since a linear velocity margin is broadened by the present invention,
the present invention solves also a problem concerning a recording
property difference caused by a linear velocity difference at the inner or
outer periphery of a disk used at a constant rotation velocity, such as
CAV (constant angular velocity) or ZCAV (zoned CAV).
Hereinafter, other components of the present invention is described
referring to FIG. 3.
In FIG. 1, examples of a substrate 1 include a transparent glass or a
transparent resin such as polycarbonate, acryl, polyolefin or the like.
Among them, polycarbonate resin is most preferable since it is inexpensive
and has been most commonly and successfully used for CD.
A lower protective layer 2 is provided to prevent deformation at a high
temperature during recording.
A material for the lower protective layer 2 is selected by considering
refractive index, heat conductivity, chemical stability, mechanical
strength, adhesion and the like. Preferable examples include generally an
oxide, a sulfide or a nitride of a semiconductor or a metal having a high
transparency and a high melting point, and a fluoride of Ca, Mg or Li. In
addition to the above materials, various materials usable for an upper
protective layer can also be used for the lower protective layer.
These oxides, sulfides, nitrides or fluorides do not necessarily have a
stoichiometric composition, and it is effective to adjust a composition or
to mix for controlling a refractive index or the like.
A dielectric mixture is preferable when considering repetitive recording
properties. Concrete examples include a mixture of ZnS or a rare earth
sulfide with a heat resistant compound such as an oxide, a nitride, a
carbide or the like. A film density of a lower protective layer is
preferably at least 80% of bulk state in the same manner as in an upper
protective layer, thus providing a satisfactory mechanical strength.
A thickness of the lower protective layer is usually from 10 to 500 nm. If
the lower protective layer is too thin, a deformation-preventing effect is
not satisfactorily applied on a substrate or a recording film, and does
not work as a protective layer. On the other hand, if the lower protective
layer is too thick, cracks tend to occur due to internal stress of a
dielectric material itself or due to an elastic property difference
occurred between the protective layer and the substrate. Particularly,
since the lower protective layer has a function to prevent a substrate
deformation by heat, the thickness of the protective layer is usually at
least 50 nm. If the lower protective layer is too thin, a slight substrate
deformation is accumulated during repetitive overwriting, thereby causing
noises by scattering retrieving light.
The upper limit of the thickness of the lower protective layer is usually
200 nm in view of a film-forming time, and if the lower protective layer
is too thick, a groove geometry tends to change on the recording layer
surface. That is, a groove depth becomes smaller than expected on the
substrate surface, or a groove width becomes smaller than expected on the
substrate surface. Accordingly, the lower protective layer should
preferably have a thickness of at most 150 nm.
The multilayer structure of the Embodiment (1) comprising a first
reflective layer containing aluminum as the main component, a
diffusion-preventing layer and a second reflective layer containing silver
as the main component, has a function of light-reflecting at a high
reflectance and a function of heat-dissipating at a high thermal
conductivity and also has a satisfactory archival stability, as a whole.
Therefore, an excellent optical information recording medium can be
obtained even by providing them on a protective layer containing no
sulfur.
Particularly, the multilayer structure comprising a first reflective layer
of an aluminum alloy containing 0.1 atomic % to 2 atomic % of tantalum, a
diffusion-preventing layer of tantalum and a second reflective layer of
pure silver, provides a satisfactory reflectance and a satisfactory
thermal conductivity stably for quite a long term without causing mutual
alloy-formation and precipitation of tantalum in the first reflective
layer.
In the Embodiment (2), the intermediate layer comprises an element which
does not form a compound with silver, the element contained in the
intermediate layer having a solid solubility of at most 5 atomic % to
silver and silver having a solid solubility of at most 5 atomic % to the
element contained in the intermediate layer and having less reactivity to
sulfur or its sulfide comprises chemically stable elements. Alternatively,
the intermediate layer comprises an element which forms a continuous
series of solid solutions with silver, or comprises amorphous carbon or an
oxide, a nitride or a carbide of a semiconductor or metal.
As this result, a change as a lapse of time of recording properties can be
prevented simply by forming the intermediate layer of monolayer.
In the Embodiment (2), a material used for an intermediate layer may be the
same as various materials illustrated as a material usable for a
diffusion-preventing layer in the Embodiment (1). That is, examples of the
material usable for the intermediate layer in the Embodiment (2) include
metal, semiconductor, metal oxide, metal nitride, metal fluoride, metal
carbide, semiconductor oxide, semiconductor nitride, semiconductor
fluoride, semiconductor carbide and other various compounds and amorphous
carbon, gold and palladium.
Examples of a metal usable for the intermediate layer include at least one
element selected from the group consisting of Ta, Ni, Co, Cr, Si, W and V.
Among them, Ta and/or Ni are preferable, and particularly Ta is
preferable. A layer containing a material which makes an alloy with silver
easily, for example, Ge or Al, can not be used as the intermediate layer,
except when it is used with other layers. Examples of a material usable
for the diffusion-preventing layer include an oxide, a nitride or a
carbide of metal or semiconductor, such as at least one component selected
from the group consisting of silicon oxide, silicon nitride, silicon
carbide, tantalum oxide, cerium oxide, lanthanum oxide and yttrium oxide.
Examples of amorphous carbon usable for the diffusion-preventing layer
include amorphous hydrogenated carbon. Particularly preferable materials
are metal or carbon.
Also, tantalum oxide has functions of both of a diffusion-preventing layer
and a dielectric protective layer. Further, in the case of a phase-change
type recording medium, tantalum oxide is preferable as an excellent
protective layer material.
When using a material absorbing laser light, such as Ta, Ni or the like, as
an intermediate layer, an optical disadvantage is provided and a recorded
signal intensity sometimes becomes low.
Also, this intermediate layer may contain other elements as far as the
addition of other elements does not substantially hinder their functions.
The intermediate layer has a film thickness of usually at least 1 nm,
preferably at least 5 nm, and usually at most 100 nm, preferably at most
50 nm, more preferably at most 20 nm. If the intermediate layer is thinner
than 1 nm, the aimed effect of the intermediate layer can not be achieved,
and if the intermediate layer is thicker than 100 nm, its effect as a heat
dissipation layer for a reflective layer tends to be poor. Also, due to a
film stress, a substrate is deformed or cracks occur. Further, it is
unfavorable in view of production cost and tact.
Other parts of the structure of the optical information recording medium of
the Embodiment (2) are described in more details hereinafter. The basic
layer structure of the Embodiment (2) is the same as that of the
Embodiment (1).
Examples of a substrate include glass or a transparent resin such as
polycarbonate, acryl or polyolefin. Among them, polycarbonate is
particularly preferable since it is cheap and it has been successfully
used.
However, when recording or retrieving by placing a head such as a float
type head or a contact type head in the vicinity or in contact with a
medium, glass or a resin having a higher stiffness or heat resistance or
metal may be used as a substrate.
Also, when recording and retrieving are not carried out through a
substrate, the substrate is not necessarily transparent.
The recording layer is usually coated with a protective layer on both
sides.
A material for the protective layer is selected by considering refractive
index, heat conductivity, chemical stability, mechanical strength,
adhesion and other properties. Generally, examples of the material include
an oxide, a sulfide or a nitride of semiconductor or metal having a high
transparency and a high melting point, and a fluoride of Ca, Mg, or Li.
These oxides, sulfides, nitrides or fluorides do not necessarily have a
stoichiometric composition, and it is sometimes effective to adjust the
composition for controlling a refractive index or other properties or to
use in a mixture. A dielectric mixture is preferable in view of repetitive
recording properties. More concrete examples include mixtures of ZnS or
rare earth sulfides with heat resistant compounds such as oxides, nitrides
or carbides. For example, a mixture of ZnS with SiO.sub.2 is often used as
a protective layer for a phase-change type optical disk.
An upper protective layer provided on the reflective layer side of the
recording layer contains sulfur, preferably ZnS, TaS.sub.2, a rare earth
sulfide or the like. Other materials for the upper and lower protective
layers may be the same as those illustrated in the Embodiment (1).
The protective layers preferably have a film density of at least 80% of
bulk state in view of mechanical strength (Thin Solid Films, volume 278,
pages 74-81, 1996).
The thickness of a protective layer is usually from 10 to 500 nm. If the
protective layer is too thin, a deformation-preventing effect can not be
satisfactorily applied on a substrate or a recording film, and therefore
the protective layer is likely to hardly achieve its function. Also, if
the protective layer is too thick, cracks tend to occur due to an internal
stress of a dielectric material itself or due to an elastic property
difference to a substrate. Particularly, it is necessary for the lower
protective layer to control a substrate deformation by heat, and therefore
the lower protective layer usually has a thickness of at least 50 nm. On
the other hand, if it is too thin, minor substrate deformations are
accumulated during repetitive overwriting, thereby sometimes causing
noises by scattering retrieving light.
The upper limit of the thickness of the lower protective layer is
substantially about 200 nm in view of a film-forming time, and if the
thickness is too large, a groove geometry is sometimes changed on the
recording layer surface. That is, the depth of the groove becomes smaller
than the aimed shape on the substrate surface, and also the groove width
becomes smaller than the aimed shape on the substrate surface. Therefore,
the upper limit of the lower protective layer is preferably at most 150
nm.
On the other hand, in order to prevent a deformation of the recording
layer, the upper protective layer should have a thickness of usually at
least 5 nm, preferably at least 10 nm. If the upper protective layer is
too thick, microscopic plastic deformations are accumulated in the inside
of the upper protective layer during repetitive overwriting, thereby
scattering retrieving light to cause noises. Thus, the upper limit of the
thickness of the upper protective layer is usually 60 nm, preferably 50
nm.
A well known phase-change type optical recording layer can be used as the
recording layer, and a compound such as GeSbTe, InSbTe, AgSbTe, AgInSbTe
or the like can be selected as an overwritable material.
Among them, a thin film containing an alloy of {(Sb.sub.2 Te.sub.3).sub.1-x
(GeTe).sub.x }.sub.1-y Sb.sub.y (0.2<x<0.9, 0.ltoreq.y<0.1) or an alloy of
Ma.sub.w (Sb.sub.z Te.sub.1-z).sub.1-w (wherein 0.ltoreq.w.ltoreq.0.3,
0.5.ltoreq.z.ltoreq.0.9 and Ma is at least one component selected from the
group consisting of In, Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Pb, Cr,
Co, O, N, S, Se, Ta, Nb, V, Bi, Zr, Ti, Mn, Mo, Rh and a rare earth
element), is preferable since it is stable in either crystalline or
amorphous state and it enables a high speed phase-change between both
states.
Further, it has an advantage that segregation hardly occurs, and it is
therefore the most practical material.
If the recording layer is phase-change type, its thickness is usually at
least 10 nm and at most 100 nm.
If the recording layer is too thin, a satisfactory contrast is hardly
obtainable and a crystallization speed tends to become low, and it is hard
to carry out recording and erasing in a short time.
On the other hand, if the recording layer is too thick, an optical contrast
is also hardly obtainable, and cracks tend to occur. Thus, a particularly
preferable thickness of the recording layer is at least 10 nm and at most
30 nm.
If the thickness is less than 10 nm, the reflectance becomes too low, and
if the thickness is more than 30 nm, the heat capacity becomes too large,
thereby likely to deteriorate a recording sensitivity.
The recording layer can be obtained usually by sputtering an alloy target
in an inert gas, such as Al gas.
The thickness of each of the recording layer and the protective layer is
adjusted in view of the above-mentioned mechanical strength and
reliability, and is selected in such a manner as to provide a satisfactory
absorbing efficiency of laser light by considering an optical interference
effect caused by a multilayer structure and is selected also in such a
manner as to provide a large recording signal amplitude, i.e. to make a
large contrast between recorded state and unrecorded state.
As mentioned above, a recording layer, a protective layer and a reflective
layer are formed by sputtering method or the like. In order to prevent
oxidation or pollution among layers, film-formation is carried out in an
in-line apparatus wherein a target for a recording film, a target for a
protective film, and if necessary, a target for a reflective layer
material, are placed in the same vacuum chamber provided therein. This
method is excellent also in the aspect of productivity.
A material containing silver as the main component used as a reflective
layer in the Embodiment (2) may be the same material as that of the second
reflective layer used in the Embodiment (1). Its thickness is usually from
30 to 300 nm, preferably from 40 to 200 nm. If the thickness is too large,
the recording sensitivity is likely to become lower because heat capacity
of the reflective layer becomes too much and a crack tends to occur. On
the other hand, if the thickness is too small, reflectivity is likely to
become insufficient and a heat dissipation effect of the silver reflective
layer is hardly achieved.
EXAMPLES
Now, the present invention will be described in further detail with
reference to Examples. However, it should be understood that the present
invention is by no means restricted to such specific Examples.
The value of the solid solubility of each element is referred to
"Constitution of Binary Alloys", (Max Hansen and Kurt Anderko, second
edition (1985), Genium Publishing Corporation, New York).
The compositions of the respective layers were confirmed by a combination
of e.g. fluorescent X-ray analysis, atomic absorption analysis and
X-ray-excited photoelectron spectrometry.
The film density of the protective layer was obtained from the weight
change in a case where it was formed in a thickness of as thick as a few
hundreds nm on a substrate. The layer thickness was obtained by correcting
the fluorescent X-ray intensity by the layer thickness measured by a
tracer.
The sheet resistivity of the reflective layer was measured by a four probe
resistivity meter (Loresta FP, tradename manufactured by Mitsubishi
Petrochemical Co., Ltd. (presently Dia Instruments)). The resistivity
measurement was carried out with regard to a reflective layer coated on a
glass or polycarbonate substrate which is an insulator or with regard to a
reflective layer which is the uppermost layer of the quadri-layer
structure as shown in FIG. 1 (before coating a UV ray-curable resin
protective layer).
The upper protective layer is an insulator and thus gives no influence on
measurement of the sheet resistivity. Further, the disk substrate had a
diameter of 120 mm, and in this measurement, it can be regarded
substantially as having an infinite area. Therefore, the measurement can
be made as it is.
From the resistance value R thus measured, the sheet resistivity rS and the
volume resistivity rV were calculated by the following formulas, wherein t
is the layer thickness, and F is a corrective factor determined by the
shape of the thin film region and takes a value of from 4.3 to 4.5. Here,
F was taken to be 4.4.
rS=F.multidot.R (2)
rV=rS.multidot.t (3)
The disk evaluations were carried out at the following conditions, unless
otherwise noted.
For evaluation of recording/retrieving, DDU1000 evaluation machine,
manufactured by Pulstec, was used. The recording linear velocity was from
1.2 to 4.8 m/s, and the retrieving velocity was 2.4 m/s. The wavelength of
the optical head was 780 nm, and NA was 0.55.
The pulse strategy shown in FIG. 2 was used for recording of EFM
(Eight-Fourteen Modulation) random pattern, wherein T is a clock period,
and in the recording mark-formation part, a recording power Pw is
irradiated into a recording pulse period and a bias power Pb is irradiated
into an off-pulse period, and the irradiations of the recording power Pw
and the bias power Pb were made alternatively, and an erasing power Pe is
applied between mark parts. However, at a linear velocity of at least 2.8
m/s, there is a case where Pb=Pe during the off-pulse period at the
rear-most end of the mark. Pb was constant at 0.8 mW at all linear
velocities.
The clock period at 2-times velocity of CD was 115 nsec. At the time of
switching the linear velocity, the clock period T was reversely
proportional to the linear velocity. The retrieving velocity was 2-times
velocity, and the allowable value of jitter was 17.5 nsec as stipulated in
the CD standards.
The recording layer immediately after the film formation is amorphous, and
is initialized by a bulk eraser. Namely a laser beam with a wavelength of
about 830 nm focused to have a long axis of about 70 .mu.m and a short
axis of about 1.3 .mu.m, was applied with an initialization power of from
500 to 600 mW at a linear velocity of 3.5 m/s to crystallize the layer
over the entire area to obtain the initial state (unrecorded state).
With this power, the recording layer is believed to have once melted and
then crystallized during resolidification.
The substrate is a polycarbonate substrate having a thickness of 1.2 mm in
which grooves having a width of 0.53 .mu.m and a depth of 32 nm are formed
at a track pitch of 1.6 .mu.m by injection-molding, unless otherwise
specified.
The groove geometry was obtained by U-groove approximation by using an
optical diffraction method. Of course, the groove geometry may be actually
measured by a scanning electron microscope or a scanning probe microscope.
In such a case, the groove width is a groove width taken at a position of
one half in the depth of the groove.
Example 1
On a polycarbonate substrate, 95 nm of a lower protective layer
(ZnS).sub.80 (SiO.sub.2).sub.20, 17.5 nm of a recording layer Ag.sub.5
In.sub.5 Sb.sub.61.5 Te.sub.28.5 and 38 nm of an upper protective layer
(ZnS).sub.80 (SiO.sub.2).sub.20 were formed, and Al.sub.99 Ta.sub.1 alloy
having a film thickness of 40 nm as a first reflective layer and Ag having
a film thickness of 60 nm as a second reflective layer were further
formed.
The layers from the upper protective layer to the first reflective layer
were formed by sputtering method without breaking vacuum, and after
forming the first reflective layer, the multilayered structure was allowed
to exposure of the room ambience for 5 hours, and the second reflective
layer was formed under vacuum again by sputtering method.
After forming the second reflective layer, a UV ray-curable resin was
coated as an overcoating layer in a thickness of 4 .mu.m by spin coating
method.
The first reflective layer was formed at a film-forming rate of 1.3 nm/sec
at a final vacuum degree of not higher than 2.times.10.sup.-4 Pa under Ar
pressure of 0.54 Pa. The volume resistivity was 92 n.OMEGA..multidot.m.
Impurities such as oxygen, nitrogen, etc. were below the detectable level
by X-ray excited photoelectron spectrometry. The sum of all impurities
could be regarded as not higher than about 1 atomic %.
The second reflective layer was formed at a film-forming rate of 1.3 nm/sec
at a final vacuum degree of not higher than 2.times.10.sup.-4 Pa under Ar
pressure of 0.54 Pa. The volume resistivity was 32 n.OMEGA..multidot.m.
Impurities such as oxygen, nitrogen, etc. were below the detectable level
by X-ray excited photoelectron spectrometry. The sum of all impurities
could be regarded as not higher than about 1 atomic %.
The film density of the (ZnS).sub.80 (SiO.sub.2).sub.20 protective layer
was 3.50 g/cm.sup.3, which was 94% of the theoretical bulk density of 3.72
g/cm.sup.3.
With regard to the above prepared medium, by employing the pulse strategy
as illustrated in FIG. 2, 2 times-velocity (2.4 m/s) recording was carried
out under a condition of Pe/Pw=0.5, and a recording power dependency of 3
T mark jitter and 3 T space jitter was shown as an initial value in FIG.
4.
In the same manner as above, by employing the pulse strategy as illustrated
in FIG. 2 except for making the clock period only half, 4 times-velocity
(4.8 m/s) recording was carried out under a condition of Pe/Pw=0.5, and
the recording power dependency was shown as an initial value in FIG. 5. In
this case, the final off-pulse period was made 0.
In either case, overwriting was carried out ten times under predetermined
conditions, and the measurement was carried out at 2 times-velocity (2.4
m/s).
As evident from FIGS. 4 and 5, at the 2 times-velocity and at the 4
times-velocity, both of the 3 T mark jitter and the 3 T space jitter had
wide power margins.
After this disk was allowed to stand for 500 hours under conditions of a
high temperature of 80.degree. C. and a high humidity of 80% RH, 2
times-velocity and 4 times-velocity recordings were retrieved, and the
results were shown as values after acceleration test in FIGS. 4 and 5. As
evident from the results shown in FIGS. 4 and 5, jitter was not
deteriorated at all in the recording power range usually used.
As mentioned above, at the 2 to 4 times velocities, wide linear velocities
and recording power margins could be obtained.
Also, repetitive overwriting could be successfully carried out about 5000
times.
Before the acceleration test, the block error rate in the CD standards of
this disk was averagely 10 counts per second, and the maximum value was at
most 30 counts per second, and this rate was not substantially increased
even after the acceleration test for 500 hours.
According to the Auger depth profile analysis of this disk, there was a
peak showing the presence of oxygen at the boundary between the first
reflective layer and the second reflective layer, and it was confirmed
that oxide layers of Ag and Al were formed at the boundary.
Comparative Example 1
A disk having the same multilayer structure was prepared under the same
film-formation conditions as in Example 1, except that all of the layers
of from the lower protective layer to the second reflective layer were
formed by sputtering method without breaking vacuum.
The first reflective layer and the second reflective layer were formed
under the same film-formation conditions as in Example 1, and had the same
volume resistivities as in Example 1.
At a 2 times velocity and at a 4 times velocity, power margins of 3 T mark
jitter and 3 T space jitter were measured, and the results were
substantially the same as those in Example 1.
Also, repetitive overwriting could be carried out up to about 5000 times.
However, after the disk was allowed to stand under conditions of a high
temperature of 80.degree. C. and a high humidity of 85% RH, the retrieved
jitter was largely deteriorated. The average block error rate was
increased more than 100 counts per second. 2 times velocity recording was
tried by using this disk, but no clear amorphous marks could be formed.
When this disk was visually observed from the reflective layer side, the
surface looked silver color at the time immediately after film-forming,
but after allowing the disk to stand in accelerating conditions
(80.degree. C./80% RH) as mentioned above, it colored slightly bluish.
According to the Auger depth profile analysis of this disk, it was
confirmed that Al alloy of the first reflective layer and Ag of the second
reflective layer were usually diffused to completely form an alloy.
Thus, it is considered that the alloy-formation between the two reflective
layers lowered a heat conductivity, and consequently the recording i.e.
amorphous mark formation could not be made.
According to Hansen's phase diagram, the solid solubility of Al to Ag is 42
atomic %.
Comparative Example 2
On a polycarbonate substrate, 95 nm of a lower protective layer
(ZnS).sub.80 (SiO.sub.2).sub.20, 17.5 nm of a recording layer Ag.sub.5
In.sub.5 Sb.sub.61.5 Te.sub.28.5, 38 nm of an upper protective layer
(ZnS).sub.80 (SiO.sub.2).sub.20 and 50 nm of a reflective layer Ag were
formed.
All of these layers were formed by sputtering method without breaking
vacuum.
After forming the reflective layer, a UV ray-curable resin was coated as an
overcoating layer having a thickness of 4 .mu.m by spin coating.
The reflective layer was formed at a film-forming rate of 1.3 nm/sec at a
final vacuum degree of not higher than 2.times.10.sup.-4 Pa under Ar
pressure of 0.54 Pa. The volume resistivity was 32 n.OMEGA..multidot.m.
Impurities such as oxygen, nitrogen, etc. were below the detectable level
by X-ray excited photoelectron spectrometry. The sum of all impurities
could be regarded as not higher than about 1 atomic %.
The film density of the (ZnS).sub.80 (SiO.sub.2).sub.20 protective layer
was 3.50 g/cm.sup.3, which was 94% of the theoretical bulk density of 3.72
g/cm.sup.3.
The disk thus prepared was evaluated, and it was recognized that both of 3
T mark jitter and 3 T space jitter had wide power margins respectively at
a 2 times velocity and at a 4 times velocity.
However, repetitive overwriting could be carried out only about 1000 times.
When visually observing this disk from the reflective layer side, the
surface at the time immediately after film-formation looked silver color,
but it was discolored after the acceleration test for 500 hours. It is
considered that Ag was reacted with sulfur of the sulfide in the upper
protective layer.
After film-formation, the disk was allowed to stand at room temperature for
several days, and as this result, the Ag film was discolored in the same
manner as above.
Example 2
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (105 nm), an Ag.sub.5 In.sub.5 Sb.sub.62 Te.sub.28
recording layer (17 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a Ta.sub.2 O.sub.5 intermediate layer (10 nm)
and an Ag reflective layer (90 nm) were formed by sputtering method, and a
UV ray-curable resin was further coated thereon as a protective coating.
After initially crystallizing this disk, recording was carried out by
forming amorphous marks in grooves. With regard to the above prepared
medium, by employing the pulse strategy as illustrated in FIG. 2, 2
times-velocity recording was carried out under the condition of Pe/Pw=0.5
and Pw=10 mW.
Thereafter, this disk was maintained under environment of 80.degree. C. and
80% RH for 500 hours, and recording was carried out in the same manner as
above. (hereinafter, operation of maintaining the disk under environment
of 80.degree. C. and 80% RH is referred to as "acceleration test".) Before
and after the acceleration test, recording was carried out, and 3 T space
jitter values were respectively 12.5 nsec and 14.3 nsec. Thus, the
deterioration degree was small. Substantially the same results were
obtained with regard to disks prepared by varying a film thickness of an
intermediate layer (tantalum oxide layer) from 10 to 50 nm without varying
a total film thickness of the upper protective layer and the intermediate
layer. Also, a signal intensity was satisfactorily high, and deterioration
was not substantially recognized.
Example 3
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.5 Sb.sub.62 Te.sub.28
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a Ta intermediate layer (40 nm) and an Ag
reflective layer (70 nm) were formed by sputtering method, and a UV
ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and recording was carried out before and after the acceleration test,
and 3 T space jitter values were respectively 15.0 nsec and 17.4 nsec,
thus proving that deterioration was small. Substantially the same results
were obtained with regard to disks prepared by varying a film thickness of
an intermediate layer (Ta layer) from 10 to 40 nm. A signal intensity was
lower than that of Example 2, but was a satisfactory level.
The solid solubility of Ta to Ag is believed to be 0 atomic %.
Example 4
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.5 Sb.sub.62 Te.sub.28
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a Ni intermediate layer (40 nm) and an Ag
reflective layer (70 nm) were formed by sputtering method, and a UV
ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and recording was carried out before and after the acceleration test,
and 3 T space jitter values were respectively 15.0 nsec and 15.0 nsec,
thus proving that deterioration was very small. Substantially the same
results were obtained with regard to disks prepared by varying a film
thickness of an intermediate layer (Ni layer) from 10 to 40 nm. A signal
intensity was smaller than that of Example 2, but was a satisfactory
level.
The solid solubility of Ni to Ag is 0 atomic % and the solid solubility of
Ag to Ni is believed to be less than 2 atomic %.
Comparative Example 3
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.5 Sb.sub.62 Te.sub.28
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm) and an Ag reflective layer (70 nm) were formed by
sputtering method, and a UV ray-curable resin was further coated thereon
as a protective coating. The same environmental resistance test
(acceleration test) was carried out as in Example 2, and there appeared
many defects observable by a microscope from the Ag side, and even before
the acceleration test, recording properties were deteriorated when
recording was repeated about 100 times.
Comparative Example 4
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.5 Sb.sub.62 Te.sub.28
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), an Al alloy reflective layer (40 nm) and an Ag
reflective layer (70 nm) were formed by sputtering method, and a UV
ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and after the acceleration test, the disk was so deteriorated as not
being capable of recording marks. According to Auger electron spectroscopy
analysis, it was confirmed that Al and Ag were mutually diffused.
Comparative Example 5
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (105 nm), an Ag.sub.5 In.sub.5 Sb.sub.62 Te.sub.28
recording layer (17 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (50 nm) and an Al.sub.99 Ta.sub.1 alloy reflective layer
(220 nm) were formed by sputtering method, and a UV ray-curable resin was
further coated thereon as a protective coating. Recording and evaluation
were carried out in the same manner as in Example 2, and recording was
carried out before and after the acceleration test, and 3 T space jitter
values were respectively 11.7 nsec and 30.1 nsec, thus proving that
deterioration was quite severe.
Comparative Example 6
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.5 Sb.sub.62 Te.sub.28
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a Ti layer (40 nm) and an Ag reflective layer
(70 nm) were formed by sputtering method, and a UV ray-curable resin was
further coated thereon as a protective coating. Recording and evaluation
were carried out in the same manner as in Example 2, and after the
acceleration test, deterioration was so severe that it was impossible to
record marks.
This was probably due to mutual diffusion and then alloying of Ag and Ti.
The solid solubility of Ti to Ag is about 5 atomic % at an eutectic point
of 850.degree. C. However, Ti and Ag form a compound of TiAg.
Comparative Example 7
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.5 Sb.sub.62 Te.sub.28
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a Zr layer (40 nm) and an Ag reflective layer
(70 nm) were formed by sputtering method, and a UV ray-curable resin was
further coated thereon as a protective coating. The acceleration test was
carried out in the same manner as in Example 2, and there appeared many
defects visually observable.
Zr and Ag do not form solid solution. However, it is presumed that Zr and
Ag formed a compound of ZrAg, probably Zr.sub.2 Ag and Zr.sub.3 Ag.
Comparative Example 8
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.6 Sb.sub.63 Te.sub.26
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a Ge layer (40 nm) and an Ag reflective layer
(70 nm) were formed by sputtering method, and a UV ray-curable resin was
further coated thereon as a protective coating. The acceleration test was
carried out in the same manner as in Example 2, and it visually appeared
that the reflective layer was browned. 2 times velocity recording was
tried by using a thin disk, but no amorphous marks were formed at all. At
a 4 times velocity, amorphous marks were formed when recording was carried
out by using shorter pulses than in Example 2. The solid solubility of Ge
to Ag is about 9 atomic % at the eutectic point of 651.degree. C.
Example 5
On a polycarbonate substrate having a thickness of 0.6 mm, 205 nm of a
lower protective layer (ZnS).sub.80 (SiO.sub.2).sub.20, 18 nm of a
recording layer Ag.sub.5 In.sub.6 Sb.sub.63 Te.sub.26 and 20 nm of an
upper protective layer (ZnS).sub.80 (SiO.sub.2).sub.20 were formed, and
Al.sub.99.5 Ta.sub.0.5 alloy having a film thickness of 40 nm as a first
reflective layer and Ag having a film thickness of 70 nm as a second
reflective layer were further formed.
The layers from the upper protective layer to the first reflective layer
were formed by sputtering method without breaking vacuum, and after
forming the first reflective layer, the multilayered structure was allowed
to exposure of the room ambience for 3 days, and the second reflective
layer was formed under vacuum again by sputtering method.
After forming the second reflective layer, a UV ray-curable resin was
coated as an overcoating layer in a thickness of 4 .mu.m by spin coating
method.
Above prepared two disks were adhered, overcoating layers facing each
other.
The first reflective layer was formed at a final vacuum degree of not
higher than 4.times.10.sup.-4 Pa under Ar pressure of 0.55 Pa. The volume
resistivity was 55 n.OMEGA..multidot.m. Impurities such as oxygen,
nitrogen, etc. were below the detectable level by X-ray excited
photoelectron spectrometry. The sum of all impurities could be regarded as
not higher than about 1 atomic %.
The second reflective layer was formed at a final vacuum degree of not
higher than 4.times.10.sup.-4 Pa under Ar pressure of 0.35 Pa. The volume
resistivity was 32 n.OMEGA..multidot.m. Impurities such as oxygen,
nitrogen, etc. were below the detectable level by X-ray excited
photoelectron spectrometry. The sum of all impurities could be regarded as
not higher than about 1 atomic %.
With regard to the above prepared medium, for evaluation of
recording/retrieving, an evaluation machine with an optical head of the
wavelength of 635 nm and NA of 0.60 was used. By employing the pulse
strategy as illustrated in FIG. 6, recording was carried out under a
condition of Pe/Pw=0.5, eight-sixteen modulation and 3 T mark length of
0.4 .mu.m, at a linear velocity of 7 m/s (2 times of DVD linear velocity).
The recording power dependency of jitter, reflectivity and modulation was
shown as an initial value (time 0) in FIG. 7.
This medium also showed jitter of less than 10% at 3.5 m/s and 13 mW by
slightly changing the pulse strategy in FIG. 6.
After this disk was allowed to stand for 100 under conditions of a high
temperature of 80.degree. C. and a high humidity of 80% RH,
recording/retrieving was carried out in the same manner at 7 m/s, and the
results were shown as values after acceleration test (time 100 hrs) in
FIG. 7. As evident from the results shown in FIG. 7, jitter was not
deteriorated at all in the recording power range usually used.
Example 6
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.6 Sb.sub.62 Te.sub.27
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), Al.sub.99.5 Ta.sub.0.5 alloy reflective layer
(40 nm), a Ta layer (40 nm) and an Ag reflective layer (80 nm) were formed
by sputtering method, and a UV ray-curable resin was further coated
thereon as a protective coating. Recording and evaluation were carried out
in the same manner as in Example 1, and a recording power dependency of
jitter, reflectivity and modulation was shown as an initial value (time 0)
in FIGS. 8 and 9.
After this disk was allowed to stand for 100 hours under conditions of a
high temperature of 80.degree. C. and a high humidity of 80% RH, recording
and evaluation were carried out, and the results were shown as values
after acceleration test (time 100 hrs) in FIGS. 8 and 9. The solid
solubility of Ta to Ag is believed to be 0 atomic %.
Example 7
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.6 Sb.sub.63 Te.sub.26
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), an amorphous carbon intermediate layer (40 nm)
and an Ag reflective layer (70 nm) were formed by sputtering method, and a
UV ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and recording was carried out before and after the acceleration test
and a change of 3 T signal and jitter values were not observed.
The solid solubility of C to Ag is 0 atomic %.
Example 8
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.6 Sb.sub.63 Te.sub.26
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a Co intermediate layer (40 nm) and an Ag
reflective layer (70 nm) were formed by sputtering method, and a UV
ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and recording was carried out before and after the acceleration test
and a change of 3 T signal and jitter values were not observed.
The solid solubility of Co to Ag is 0 atomic %.
Example 9
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.6 Sb.sub.63 Te.sub.26
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a Cr intermediate layer (40 nm) and an Ag
reflective layer (70 nm) were formed by sputtering method, and a UV
ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and recording was carried out before and after the acceleration test
and a change of 3 T signal and jitter values were not observed.
The solid solubility of Cr to Ag is 0 atomic %.
Example 10
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.6 Sb.sub.63 Te.sub.26
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a Si intermediate layer (40 nm) and an Ag
reflective layer (70 nm) were formed by sputtering method, and a UV
ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and recording of amorphous marks was capable of being carried out after
the acceleration test, while a change of reflectivity was observed.
After the acceleration test, there not appeared an alloy forming of the
reflective layer with the intermediate layer.
The solid solubility of Si to Ag is 0 atomic %.
Example 11
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.6 Sb.sub.63 Te.sub.26
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a W intermediate layer (40 nm) and an Ag
reflective layer (70 nm) were formed by sputtering method, and a UV
ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and recording was carried out before and after the acceleration test
and a change of 3 T signal and jitter values were not observed.
After the acceleration test, there not appeared an alloy forming of the
reflective layer with the intermediate layer.
The solid solubility of W to Ag is 0 atomic %.
Example 12
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.6 Sb.sub.63 Te.sub.26
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a V intermediate layer (40 nm) and an Ag
reflective layer (70 nm) were formed by sputtering method, and a UV
ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and recording was carried out before and after the acceleration test
and a change of 3 T signal and jitter values were not observed. After the
acceleration test, there not appeared an alloy forming of the reflective
layer with the intermediate layer.
The solid solubility of V to Ag is believed to be 0 atomic %.
Example 13
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.6 Sb.sub.63 Te.sub.26
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), an Au intermediate layer (40 nm) and an Ag
reflective layer (70 nm) were formed by sputtering method, and a UV
ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and recording was carried out before and after the acceleration test
and a change of 3 T signal and jitter values were not observed.
After the acceleration test, Au and Ag may form a solid solution. However,
Au and Ag form a continuous series of solid solutions, an do not form any
specific segregation, which results in no significant change of
reflectivity and thermal conductivity.
Example 14
On a polycarbonate substrate, a (ZnS).sub.80 (SiO.sub.2).sub.20 lower
protective layer (95 nm), an Ag.sub.5 In.sub.6 Sb.sub.63 Te.sub.26
recording layer (18 nm), a (ZnS).sub.80 (SiO.sub.2).sub.20 upper
protective layer (40 nm), a Pd intermediate layer (40 nm) and an Ag
reflective layer (70 nm) were formed by sputtering method, and a UV
ray-curable resin was further coated thereon as a protective coating.
Recording and evaluation were carried out in the same manner as in Example
2, and recording was carried out before and after the acceleration test
and a change of 3 T signal and jitter values were not observed.
After the acceleration test, Pd and Ag may form a solid solution. However,
Pd and Ag form a continuous series of solid solutions, an do not form any
specific segregation, which results in no significant change of
reflectivity and thermal conductivity.
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